Women will go for taller, men will go for slimmer, so the shorter and fatter sorts will lie.
It's a while since I've online dated but I do remember putting a full-body shot up specifically because I kept being asked for one when I got into conversations with guys - presumably in a "making sure she's not deceptively twice the alleged size" kind of way.
Don't get why people bother if the objective is to pick up dates, it's not like you can keep up the lie in person!
But I think a lot of people won't reveal all about themselves in a profile pic...or their bio.. as they feel slightly vulnerable... putting it all out there to be judged by strangers and to be found not good enough....
Society tells us we must meet a partner the old fashioned way...to use a dating site in some people's eyes is an admittance of failure of conventional norms ( more so in years gone by thank feck) but perhaps some still feel the stigma... dunno just spitballing