I think w/ breeches, it's a little more subjective than a jacket. For example, some prefer a loser fit, while others like them tight. The material you buy makes a differnce too. For example, my four-way stretch Tailored Sportsman breeches fit a little more liberally than the two-way.
I prefer my breeches to be low rise w/ a tad of room through the hip, not not sloppy. Similar to the fit you see in the Smart Pak banner at the top of this page.