Well, being among the ‘non-Twiggies’ - even when I weighed a LOT less than I do now - I will tell you what I think looks most attractive.
First: horse has pattern - you do not need or want to compete
Second: darker colors (not necessarily black); solid colors (perhaps a tone-on-tone vertical stripe if you absolutely cannot stand not having SOMETHING; NO BLING
You have a black felt hat and black boots. What color chaps? Black, I hope. So build your outfit around those expensive items. Assuming black chaps, any color other than black for pants is going to be an attention-getter, since that color will be framed by your chaps - front & back. You don’t really want a loose top (or pants for that matter). Fitted enough so it (pants or top) doesn’t wrinkle - it’s a fine point, but baggy accentuates weight as much as skin-tight does. You don’t want sloppy or sausage.
If you’re in a warm-weather area, for the top, I might try something like a black sleeveless stretchy top (more than tank top, more like a scoop neck ladies’ t-shirt sort of thing - I’ve seen what I mean in WalMart/Target) with an attractive, ‘softer’ look long-sleeved shirt almost tunic-length top with ‘give’ - and perhaps either tone-on-tone stripes as I suggested before (I’ve seen sort of matte & glossy, about 1/2" wide) or even very thin lines of one contrasting color. Silver/pale gray/pale blue.
Oh - and a tastefully sized buckle on a black - no bling - belt.
Whatever, make sure you try it on ON the horse to make sure it allows for all necessary movement - including climbing on and off! Move your arms forward - you don’t want it to pull across your back.
And SUPPORT!!! All-important.
Carol