Not uncommon to have a girth loose on the sides and tight on the bottom. Has to do with the way the horse is shaped. There are some horses shaped so the girth grips all the way around, and others where it primarily grips on the bottom. I have had horses that would develope swelling from too tight a girth (and they were shaped like your horse). I solved the problem by going with a professional's choice girth; the neoprene lining made it less slippery so I didn't have to tighted it as much, and the double elastic gave it enough give that it wasn't too tight. Worked well for my horses.
I have the same problem with my slab sided mare, but I only experience it with my ergonomic girth. So, I don't think I'd recommend one. In my experience the ergonomic girths tend to be a little stiffer (although I'm sure there are all varieties).
I had a slab sided TB like this. For him, the key was not over tightening. He had a massive set of withers which kept side to side motion at a minimum, and I used a breast plate and an anti slip pad to keep the saddle from slipping back. From there, I kept the girth snug, but never super tight.
Thank you for this!
I prefer snug as well. My trainer is the one who always comes over and cranks my TB's girth up a hole or two and I can see my poor guy's eyeballs bulge out like "Are you kidding me???" She and I also disagree frequently on where to his saddle should sit. I think she greatly underestimates how high his withers are. I'm neurotic and research the h*ll out of anything before I buy it to make sure it fits my horse well, so if the saddle fitter says my saddle fits well and it's in the right place (and it doesn't move around with a snug girth), I think I deserve a little credit for doing right by my horse.
Have you ever tried a humane girth? I think they are way more popular in the UK than they are here (I can't find them on US tack websites but a lot of the UK ones carry them, as do many sellers on ebay). They prevent overtightening (well, the all-leather ones, they do make ones with elastic inserts now) and ensure even girthing. Most of them are fairly reasonably priced (even the padded leather ones). So, here is an example: http://www.rideaway.co.uk/aerborn-waffle-humane-girth
But I've seen them in leather and nylon as well from different sellers, if waffle isn't your style.