I haven't been able to find anything that tells me how big your belly is supposed to be exactly. While I can get a general idea from the description in character attributes, I'm more curious as to how big my Steele is getting exactly
Belly size bonus rating? The 'belly' section is under vagina sections, and indicated there are the total sum of bonuses from pregnancies and cumflation.
In the save editor, you can see the base rating (usually 0), and pregnancy bonuses (they are indicated aside, added up). Cumflation is stored separately somewhere else.
I found a bug of Venus Pitcher pregnancy which decreases the total preggo bonus, it's now treated by Jacques00.