[warmachine6]

Scale is erm not the right word here its the word height,
i.e we wouldn't want a 6 inch high hulk-buster, in all fairness they're all out of scale otherwise all our figures would be 6 inches tall.

Anyway.....

Now lets say ML's Hulk-buster isn't the correct height for ML's 6inch Hulk.
I would look at the movie and see how the height difference is to the film, I would imagine that the ml's hulk-buster is in scale with ml's hulk, it maybe slightly shorter in the film, visa verser, to me they share the same height i think.

i do imagine hulk needs to be taller than most figures , its down really to research and how much accurate you want to be.
i.e the hulk buster is taller than hulk by a few inches, buy the hulk figure. wait for reviews of two different hulk buster to which people hopefully would have had the same hulk figure to do size comparions, so you can get your satisfied hulk buster to fight hulk