But as a spider morph, yeah, it was a good chance she could have a wobble while settling in. And as was mentioned, there's a whole spider complex of genes that can cause wobble. Spiders are just the one that people pile on the most.
Being a bright yellow, I would agree with the possible bumblebee (spider/pastel) morph... But again, axanthic tends to photograph terribly so without me seeing it in person it may be hard to tell on that.
I honestly don't think she looks that bad sizewise. She looks a bit more defensive, so if she's really trying to show off, sometimes the way they puff up can make them look underweight because of how the spine looks.
She may have been a late start on feeding. I have a girl that has only gained 15 grams since July. And weighs in at around 40 right now. Another girl I picked up was a year old and weighed around 350. She was just on a maintenance diet with the breeder and not a growth focused one. So she ate about every 7-10 days.
I would consider getting the snake sexed as well since the morph wasn't clear, that may have been mislabeled as well...the size could also be an issue with her actually being a he. =\