OP, my DD was big (not quite this big, thank goodness), so I have some experience with this. You've just gotta get creative.
Sleeveless shirts with a little sweater on over it (sleeveless always fit better than sleeved when you're a fat baby!).
Old Navy sells cropped leggings- my DD was living in 2T and 3T in those at a year old. They are awesome for the fat ones because they give them the waist, thigh, and leg opening room, but they're shorter. I got ours at the outlet mall at Anne Arundel- the website has these
http://oldnavy.gap.com/browse/product.do?vid=1&pid=150982332 but they're almost sold out. Must be a summer thing, unfortunately, but I'd look around and see if you can find some in another brand.
Get pajamas without footies- that way you can fold up the legs and stick some socks on her.
And say goodbye to the Bumbo. By the time my kid was old enough to sit up in it, her thighs were too fat to fit. Be warned. She's now just 2 and is creeping into the 4Ts. My best advice is to only buy a small number of clothes in each size. The big babies blow through them too fast!