Vildy and JP are correct—the major problem is with the cut of the crotch—and Angie is right that size just aggravates the problem.
As someone who used to do a lot of sewing and tailoring, the key to figuring out pant-fitting issues is to follow the crease lines to the fit areas which are the source of the fitting issue. The rule is bodies are never the problem, so fitting is always about the cut and construction of the garment.
Crotch construction in women’s pants is inherently complex because we need to fit two-dimensional fabric components (length and width) at a critical 4-point intersection on a three-dimensional body. So, for what’s it worth, my first instincts as to the possible issues with these pants are as follows:
(A) the inner seam of the front leg needs to be released and the crotch point moved forward to allow enough room to sit in the pant. Scooping out more room in the rear seam curve might give enough ease to allow this to work. Letting out the top inner leg seams might also add a bit more ease—even a 1/4 of an inch can work wonders here.
(B) The front part of the crotch seam is far too long because your flat stomach doesn’t require the fabric to accommodate a curve in this area. Scooping out the extra fabric length at the point where the front crotch seam meets the waistband and then readjusting the waistline curve to the side seam could help reduce that extra fabric which gets pushed down into folds when you sit.
The fix, as I see it, would be to remove the waistband, fit the front and rear crotch seams to your actual waistline and crotch intersection, adjust the side and inner leg seams, and then refit the waistband so it sits comfortably on your waist when standing and, importantly, when sitting. A reasonably skilled tailor could handle the job, but, if you understand what is involved, you can steer clear of anyone who just wants to do a half-assed job of quickly running another seam along the back crotch seam. That might move the crotch intersection forward, but won’t fix that roll in the front or help the pants feel more comfortable when you sit down.
