Actually, the very nature of the theatre business means that it makes little to no sense to try to compare one show's grosses to another. Shows can have dramatically different running costs - consider what it must have cost Venus in Fur to run compared to the running costs of Spider-Man. Therefore, it's entirely illogical to say, "Spider-Man outgrossed Venus in Fur so it must be doing better!"
That was kind of an extreme example to prove a point but even musicals can have dramatically different costs based on how big the names are that need to be paid, a larger orchestra, etc. Furthermore, shows play in houses with very different capacities. Unlike most corporations which theoretically have unlimited potential market share if they globalize, any single Broadway show in a given week is limited to however large its house is.
It's not a feat or a surprise that a show at the Foxwoods (15,440 seats per week) would outgross a show at the Helen Hayes (4,664 seats per week.) Every theatre has a different capacity and prices and meets a certain gross as a result.
None of this is to say that grosses have no value but rather to say that a direct comparison of grosses tells you little to nothing about how well a show is doing. There are many other factors at play here and there is also something to be said about a show really competing against itself first and foremost. If a show is meeting (or better yet, exceeding) its weekly costs, it will continue to run until it becomes profitable - the ultimate goal. Of course the producers of every show want to maximize profit, but they don't necessarily care if their profit is perhaps a few percentage points less than that of another show if they too are earning profit.
By the way, I'm not picking on Spider-Man. It's just a very extreme example of a show with high running costs and high grosses.
Scratch and claw for every day you're worth!
Make them drag you screaming from life, keep dreaming
You'll live forever here on earth.