The saddle is a bit small on her and a bit small on you. I am also a heavy rider and saddle fit/comfort for you is very important. That saddle will cause you discomfort at the trot and canter, most definitely. You want at least 3 fingers of smace behind your rear and the edge of the cantle. I have a good sized belly myself, so the front room doesn't always work out, , but room behind you is important. That lets you know if you have a properly fitting saddle. I would look in to a 19-20" Duett saddle. The Duetts come in larger seats (though they do run a tad small in fit compared to others) and they have trees that work well for roly poly horses. I ride in a 19" Stubben AP (17" Western) and the 19" Duett was a bit more snug feeling than my Stubben.
I would work on lunging her daily and take her in-hand on "walks" until she loses a fair amount of weight. I would also evaluate her diet to make sure it's as calorie reduced as possible while still allowing for proper gut health/function and nutrition. Once she's nearly what you want weight-wise, call up Duett and get a demo saddle to try out for you both.
I was able to pull weight off my mare over the last 3 months by restricting her hay to only what she needed (2.5% of her target weight in bermuda hay). Her "feed" consisted of a vitamin/min supplement and 1/2 cup of flax meal mixed with 1 lb of chopped alfalfa hay and 1/2 cup of apple cider vinegar. Her pasture is overgrazed, so she only had a little grass to nibble on to keep her busy. I'm happy to say she's almost ribby! Lol I prefer my horses "lean" with good muscle. She's now back on free choice hay (round bales) and riding 4 hours a week, but with no increase in her "feed."