Really depends on the character and the extent of the personality change. Like leaning into parts of a character’s personality as they gain makes sense and is good. I think personality should change to a certain extent. But if you make a typically nice character 700lbs or w/e and totally bratty, I lose all interest. So it’s all about context at the end of the day for me.

For funny reasons, he should have bigger tits bigger than Goldmary at similar weights.
That aside, I think they're probably pretty big compared to a good chunk of the feminine cast's chests. I think he's got a fairly masculine shape, very belly and tits heavy. That top of his in his casual outfit does not leave much to the imagination at big sizes I'll tell you what.

I don't really think abt u's that much tbh but
skinniest Rin b/c idk she just doesn't have the vibe
fattest is either Umi b/c irony is funny or Nozomi b/c I'm basic

Final Fantasy 16 but everyone is fat (this does not affect eikon fights)

I think Ivy should be tbh.
She's royalty from the winter nation and she's built like that?? Nahhhh she's got a tum
For a non-Engage answer, I think Lissa should be chubby, as a treat. Or Nephenee.

Short answer: Probably fat.
Longer answer: It kinda depends on who her mom is tbh.
Like a Sumia-mothered Lucina is gonna be big.
Sully or Robin I don't really see it as much. Kinda depends on how someone interprets the character in the latter case.
Maribelle and Olivia I can kinda go either way on.

To start off, Goldmary already dislikes fighting to begin with, so I doubt she would at 400lbs or w/e, even if she was asked.
Yunaka I feel like is Bob from Tekken, and is just like acrobatic af regardless of the fact she's fat as fuck
Etie will somehow lose all of it in a matter of days and be back to her muscular form.
Ivy's sillier tendencies would increase, but she'd be fighting

This concept is fun, but I feel like at a certain point it becomes fat fetish spy vs spy lol
Like they start doing wacky crazy shit that somehow goes unnoticed.

Micaiah feedee - Kiran feeder
Edelgard feedee - Kiran feeder
Corrin feedee - Kiran feeder

I mean there're a lot of answers that just basically boil down to "popular character from popular media" like Tifa or Aerith, or <insert popular seasonal anime character (Yor, Fireren)>
This applies to FE too, in a way, there's a lot of Camilla, and other generally popular FE girls.

Not sure on the obscure part, but Ophelia Octopath Traveler would be cute chubby I think :)

The Aqours elf and rocker sets are and always will be peak. As far as SIFAS, honestly it'd mostly be less idol outfits and more the cozy unidolized outfits lol. Like the one event UR Riko where they're seeing cherry blossoms at night, where's she's got that cardigan or w/e...peak.

I'm biased, but I think Charlotte has a strong shot at it. Camilla, unfortunately, also a strong chance. Felicia and Flora also could be pretty big, I think. Rinkah is somewhere in there too

5 guys: Diamant, Chrom, Pandreo, Rosado, Kaze
5 girls: Cordelia, Charlotte, Corrin, Goldmary, Yunaka

Severa I guess would be the one that comes to mind the most. Generally the more tsundere they are, the more likely they are to deny it. Someone like Etie might, but her bluntness I feel makes her more freely admitting to it.