She looks to be exactly the same size as my new boy Dee Dee Ramone, who I adopted at barely 4 weeks old. A couple I met had driven 3 hours to rescue 3 clutches of eggs that a breeder was going to kill because he decided he was done with breeding (should have figured that out BEFORE breeding them). He was sooooooooo tiny when I got him, I was worried that he would have issues, but I took him early because this couple ended up with over 70 baby beardies at one time and we're going broke. Now Dee Dee is just over 3 months and is growing like a weed and perfectly healthy, but he has the biggest, fattest belly I've ever seen on a baby/juvenile beardie...Until seeing your guy, lol!
I feel a little better, though just yesterday he was sitting on the couch with me watching TV, and I swear I looked over at him and said "Oh my god Dee Dee, your legs and head are shrinking!!!" I think their legs (and head in Dee Dee's case, ?) just look small because you're looking at them in proportion to his big belly. His legs aren't actually small, they're just small compared to his big, round belly! I'll take a photo of Dee Dee and post it here in a minute so you'll feel better, I'm pretty sure that Dee Dee is fatter than your little guy...I don't know why, I've fed him exactly the same diet I fed my prior male who lived to be just under 13 and my current 9 month old female, and neither of them were ever overweight, if anything my girl started out a bit underweight until she had a massive growth spurt at around 6 months old. I was actually worried about her having parasites at one point until her fecal test was negative.
I think your guy is fine, again his legs look too small only because you're looking him as a whole, and his legs are out of proportion to his big belly...What is his diet? I'm just trying to compare to make myself feel better about my little fatty, lol..
I'll take a pic in a minute...