If that is the case, I recommend LittleMsSam's pregnancy overhaul, specifically the smaller pregnancy options, which should allow you to see more of the larger morph differences, without changing anything hardcoded. The script override should have no ability to harm your game, however if you don't feel comfortable with digging through your game's files just for a mod, I support that. I tried to create a workaround without anything advanced, but I only recently got into scripting for The Sims 4 and tried the best I could. I have made a script override workaround for this and included it in the 2x version, however it requires an advanced installation which is detailed in the readme. (Depending on your mods, the larger size morphs may not be visible because of a hardcoded limit.

If there are any bugs, please, let me know, I may be able to fix it or at the very least, help. If there is a mod that is incompatible, please let me know. + Since it is a script mod, it should be compatible with most if not all mods currently available for The Sims 4. Defaults to 25% and can be customized via command. im also not sure if the belly slider effects pregnant bellies as i dont currently have either a belly slider or a pregnant sim. but youd have to do every stage and every pregnancy individually. If set to 50%, it is applied per baby, meaning 50% with triplets will be 100% larger, and 50% with twins will only be 50% larger. adjusting the belly in CAS is currently possible with a belly size slider and mods like Master Controller that allow you to access CAS in-game. Twins and triplets will look larger ranging from 0% to 50% size gain per baby. + Multiple pregnancies will no longer look the same as singles. Defaults at 15% randomness and can be customized via command.

Customizable length randomness ranging from 0% all the way to 99%! Though, 99% means some unlucky sim will be pregnant for a very long time. + Some pregnancies will last a little longer than normal, others will last a little shorter. This mod is inspired by a mod for TS3 with the same name, and it adds just a little bit of realism. Add some variance to your sims' pregnancies with this mod right here folks.

Sims 4 belly size mod