Margo is right
The mega musical term was used to describe the big overblown spectacles that were imported from here in the UK (sorry lol)
In the 80s it seemed anything with a lavish set, big effects etc was a hit here in the UK.
Les Mis, Miss Saigon, Phantom, Cats, Starlight Express etc etc.
Sometimes the critics hated them here in the UK (les mis was hated by the critics when it first opened) but new theatre goers and tourists flocked to see them because of the gimmick (helicopter, Barricades etc) and because the logo's seemed to be everywhere.
They all seemed to be MASSIVE hits here in the UK.
Carrie was also supposed to be a mega musical with its special effects and clever set design (unfortunately its clever set design was the shows biggest problem and it flopped in the US despite been well received by audiences here in the UK and getting some positive notices)
I have an article from 87 talking about the "only 2 shows that mattered in the 88 season would be 2 mega musicals Carrie and Miss Saigon"
Gone With The Wind was another MEGA musical here in the UK that did not work (the tourists kept it running for almost a year)
The US embraced a lot of the Mega Musicals from here but also got wise to the fact that shows like Starlight Express just was not very good (yeeeah well done you)
The American press used to hate our musicals for been all budget and no heart.
Ironically now the US seems to have gone in to the business of mega musicals like Wicked, Pirate Queen and disney shows etc and the UK is now the ones sick of the Mega American Musicals like Wicked etc.
Lion King has been a hit here where as Beauty and The Beast did not do as well and Wicked was critically hated here and rumor has it the show is not doing as well as expected.
Namo i love u but we get it already....you don't like Madonna