Have you played TLD for the original game by any chance? This fix can be used for that too, if your interested. I've done this for both Warband and the original M&B and can run larger scale battles much better, as well as missing out on all the memory crashes
Anyway, original source would be this
but I'll summarise it here
1) Download CFF explorer
2) Create a backup of your mb_warband.exe file
3) Right click on your mb_warband.exe > Open with CFF explorer
4) File header > Characteristics > Click here > and tick "App can handle more than 2gb address space, and save/overwrite if it prompts you to
Can't guarantee it will fix your problem, but worth a shot.