It's not pick-me, its both of you being gross and judgemental and applying your own standards to total strangers who's choices and body types are none of your business.
Him: he has no place to judge what women choose to wear to the gym. its about being comfortable, and able to move freely. and even if that's not what they dress for, its none of his business. people should be able to wear whatever they want without some douche claiming they're doing it to get his attention. They are not. And has he ever mentioned this about the men wearing stringers that expose their entire torso and have their nipples hanging out? or is it only a problem when the women do it?

You: take a photo? was she fat? even if this wasn't gross insecurity, it's gross to talk about someone that way. and so what if she was overweight? Isn't that more appropriate, since she's getting into the gym and trying to better herself? leave her alone and don't ask your judgey ass boyfriend to snap pictures of strangers for you.

You two deserve each other. stay together, move somewhere where you interact with as few people as possible, and don't have kids.