Trucha Signer Error - Can't Replace Fast File

I'm wanting to use Simon's ultimate edit of Nity's mod menu on BO.

The file size is 572kb. Trucha doesn't let me override the original file size which is 571kb.

I found this mod isn't the only one with a size of 572kb. I also can't manage to edit the mod and remove any files from it... not that I really want to in the first place.

I'm sure there's a simple explanation or something I've missed. It's been many years since I've done mods on this game

You said it isn't letting you replace the .FF file... Is there an error that Trucha Signer gives? Do other mods work without any errors? You mention size... Have you ran into an error before, regarding file size? If so, perhaps, experimenting a little bit with adding or removing items from the mod will, at best, make the mod work, and at worst, give you an error, which only further helps you understand what is going on. The more errors you get with trying different things, the more understanding you will have how the program works... What works and what does not.

I am sure there is a simple explanation as well, however, you haven't given much explanation to what exactly is going on.