Before I get into my take on why I must disagree with previous posters who labeled the Next to Normal a Mid-Scale Musical. Below are in my opinion the difference between Small, Mid, and Large sized Broadway musicals.
Small sized Musical: Generally have casts of less then 10 members(excluding understudies). Small Sized musicals generally don't require an chorus, and if a show has one, its usually a small 3 person Greek-styled chorus.The orchestra requires less then 10 musicians (this does not include a synthesizer being used to replace other instruments).
Examples: Next to Normal, Title of Show, Fun Home, The Last 5 Years, Avenue Q, Little Shop of Horrors.
Mid-Sized Musicals- In my opinion is what most modern musicals are. They have cast of about 10 to 30 members. The orchestra pit is generally made up of of less then 27 musicians. In the current climate I would actually reduce the pit to under 20 musicians.
Examples: Hairspray, Hamilton, Spring Awakening, Jersey Boys, Chicago, Waitress, Newsies, Once,
Large Size Musicals- This is what most of the classic musicals of the golden era fall into. They generally require a cast with more then 30 members and a pit of 27 or more musicians.
Examples include The King and I, On the Town, South Pacific
**There is a difference between a musical's written size and a production of that musical's size. They can easily be defined by their capitalization and weekly running cost, but below is my breakdown of that that their capitalization and weekly running can be made up of. Please note that these numbers are for new productions as producing cost have risen over the years.
Small Scaled Production- Has a capitalization of less then $7 million and/or a weekly running cost under $400k. This is generally made of a a cast of under 15 members, with an orchestra of less then 10 musicians. Uses a unit set or a bare stage with a the occasional set piece, and props to define a scenes location. Uses generally basic costumes, or has the cast in the same costumes throughout the entire production.
Mid-Sized production- Has a capitalization between $7 million and $12 million and/or a weekly running cost between $400k and $750k. The cast with less then 30 members and has a pit with less then 27 musicians. They use either a unit set, or sets that use a few pieces to define a set location, instead using grandiose sets that completely transform the set from one scene location to another. In regards to costumes, you would not generally regard the the costumes as a whole as being extravagant, their may be extravagant costumes during a particular scene though.
Please note that a production can still be mid sized with if it has extravagant costumes or a grandiose designed set, as long as it does not have both. For example Hamilton has what I would consider to be extravagant period costumes, but utilizes a basic unit set. On the other end of the spectrum the 2016 Roundabout revival of She Loves Me, had a very grandiose set, but rather simple costumes. Both of these productions would be considered Mid-Sized.
Large Scale Productions-. Has a capitalization greater then $12 million and/or a weekly running cost over $750k. Generally have cast of more then 30 members, and have an orchestra of 27 or more musicians. Have several grandiose sets and costumes. The production uses special effects such as wire work for flying, pyrotechnics, or falling chandeliers, flying/levitating vehicles....
As I noted previously a musical's written size and production size can be different below are different examples of these combinations in my opinion.
Small Musical/Small Production- Next to Normal (Original Production), Title of Show (Original Production), Fun Home (Original Production),
Small Musical/Mid-Sized Production: Avenue Q (Original Broadway Production), Little Women (Original Broadway Production), Little Shop of Horrors (2003 Broadway Revival)
Mid-Sized Musical/Small Production- Godspell (2011 Broadway Revival), Spring Awakening (Original Broadway Production), Once (Original Broadway Production), The Color Purple (2015 Broadway Revival), Sweeney Todd (2005 Broadway Revival), Chicago (1996 Broadway Revival)
Mid-Sized Musical/Mid-Size Production- Newsies (Original Broadway Production), Hairpray (Original Broadway Production), Hamilton (Original Broadway Production), She Loves Me (2016 Broadway Production), Sweeney Todd (Original Broadway Production), Chicago (Original Broadway Production)
Mid-Sized Musical/Large-Size Production- Beauty and the Beast (Original Broadway Production), The Lion King (Original Broadway Production), Phantom of the Opera (Original Broadway Production), The Color Purple (Original Broadway Production), Shrek the Musical (Original Broadway Production), Miss Saigon (Original Broadway Production), Wicked
Large Musical/Mid Sized Production- Ragtime (2009 Broadway Revival), R&H Cinderella (Original Broad Production),
Large Musical/Large Production- The King and I (2015 Broadway Revival), South Pacific (2008 Broadway Revival), On the Town (2014 Broadway Revival), Spiderman Turn of The Dark,
Next to Normal, is a small scale musical and unless a director and producer have a lot of money and really want to build a replica House on stage alla August: Osage County, it will be a small-scale production as well. The show only requires a cast of 10 including understudies, and the original production's orchestra was only made up of 6 musicians. Yes the original set was 3 tiered stories, but it was bare bones being made up of industrial piping, glass panels, and incandescent looking (LED) light bulbs, but was essential a symbolic unit set and not a literal one.
Back to the topic though as previous posters have said there isn't much of a market in the West End for Small-Scale Serious Musicals. The West End Musicals that succeed are more often then not crowd pleasers with great production values. British audiences do not seem to see the musical as an equivalent to a straight play in terms of artistic value. They see musicals as solely as pieces of entertainment, where you can turn your mind off and enjoy for 2.5 hours. This is why many fear that Hamilton will not be as big a hit in the West End as it is on Broadway as not only is it about American History, but requires its audience to think and actually ask some tough questions on Race, immigration, and political purpose.