i dont think so.. i think QH breed horses, even crosses (like apendix, exc) are typically more round. i think she looks nice. but then again im parshal to "hefty" horses.
is she out to pasture?
does she ever get excersize?
how much grain and hay are you feeding her?
does she get wormed on a regular basis?

with our stalled horses (in florida which makes a difference) they get one scoop of grain, two pads of t&a hay, worked daily and wormed monthly (or daily depending on brand)

talk to your vet.. but every horse is different and every situation is different. if shes getting a reasonable ammount of grain and hay without over feeding i wouldnt worry, shoot i had a thoroughbred who i couldnt put weight on no matter what i did.. i would LOVE to have an "over weight" horse lol. good luck, hope that helps