She may have actually qualified as obese at times, as there's an actual medical classification.
I don't think that's likely to be the case in that second photo you shared, she does have a heavy build in general, but there have been times since being on ST when she likely did qualify.
IIRC, obese is anything over a specific rage of BMI or BFP, whichever you use. So if a healthy BMI for you is 18-25 and you are 30 then you are obese.
I'm not a pro, so I'm not sure about the hard details, but the point I'm making is that obese isn't a personal opinion of looks too fat. A person doesn't necessarily even need to look it, but they could still qualify.
My SO looks fine at 30, but not much above that. I think they're sitting at 28 currently.
She's the ships counselor, who at any moment may need to have deeply personal and unofficial private conversations with anyone in need. Those discussions are officially informal. Thus she maintains a casual professional appearance.
It's not jazzersize, it's public lounge wear, she is at ease to help aid her clients ability to transition to an at ease state.
While doing official duty on the bridge it is appropriate for her to dress in uniform, but those duties tended to be momentary, not planned, so a wardrobe change would've been an odd choice.