Yeah, my primary project is near that size, and I've never had anything close to a memory problem. It's amazing how tightly VB6 code compiles, even for hundreds of thousands of lines of code.

Personally, I think Dilettante nailed it, you've got some kind of recursion problem that's overflowing your stack. This could be a simple recursion problem, such as some procedure making calls that eventually re-call that procedure. Or, it could be something more complex, such as some code in a class (or form, or UC) that's somehow getting into a loop where it's making unlimited instantiations of itself.

Either of those will produce your symptoms.