I would strongly suggest using VMMap.exe from SysInternals/Microsoft, pick your existing/running mirc32.exe process (when its bloated), and provide a screenshot of the results. Please be sure to resize the window horizontally so that all the columns in the top section of the program (e.g. Type, Size, Committed, etc.) are visible.

The answer as to what's allocated all the memory may or may not be easily determined from use of this program, however. Heap and stack allocations are "vague", meaning there is no easy way to say "this specific script/yadda yadda is causing this allocation problem". However, if your scripts rely on DLLs, there is a good chance that DLL will show up in the list of allocations near the bottom under "Details" (have fun scrolling through all the data).