Hi, in AC 25 there is a difference in the way how .mod file is saved when you use "File/Save as" or if you use Publisher sets. In first case it saves also hiden layers and in second one it saves only visible layers. May be this is the case but I have not tested it in AC24.
Other explanation can be that it was saved with different settings in .mod save options.
From my knowledge, the module file size increase may happen is due to the Classification items in the MOD file. For example, if you create an expression, and assign all Classifications to it, the file size will increase due to the exported classifications.
It can be verified in the Classification Manager. Could you please let me know if this is the case?
If that's the case, would it be possible for you to share the original PLN (from both AC24 and 25) with us for in-house testing? If yes, please let me know if you will need a secure place to upload the file(s).