honey, there are men trainers out there that weigh more than you, riding horses not near as stockily built- just fine might I add. So you carry the weight differently than they do. As long as you are fit enough to get the job done I say go for it.
I ( pre preg)weigh 160-165 (don't look like it, Im built like a brick sh*t house- been that way since puberty) on a 5'4" frame...add to that the weight of my saddle/gear (approx 40 LBs)...you do the math. I ride a 14.1 QH who I wouldnt consider overly stocky. She can pack my butt all day long- heck she can carry my 190 lb hubs+gear all day long no problems.
Also Had an arab cross mare (for 13 years) who was dainty (approx 750 #) and she had no problems with packing me around either AND I rode her hard and long in her younger days...shes living with the older couple I previously boarded with as she and the b/o wife are just two peas in a pod- they spoil the crud out of her, and continue to ride her (b/o w is no skinny mini either) as she is/stays perfectly sound/happy and she's nearing 30.
I am in the minority that thinks that people just don't give the horse enough credit on what they are capable of. Horses should be treated on an individual level as far as carrying weight. Ride, assess and if the horse is doing fine, carry on.