Yes to both. The measurements for the gullet and the bars are measurements designated to fit the horse, the seat size is what fits the rider. Western saddle's seat sizes usually run about 2 inches smaller than english saddles, though depending on the style of saddle you want and what is comfortable for you, it may not stay at 2 inches.
If you are buying for a horse you already own, then I would suggest that you find a saddle fitter to come out and help you decide what size you'll need. There are a ton of considerations when buying a western saddle that will help to ensure a good fit.
It depends on you, the horse's body shape, the the maker/brand, and your saddle type preference. I like barrel saddles and ones like them because they tend to be lightweight, have a high, supportive cantle, and give you a nice, good deep seat.