Have a look at how the creases behave.
If they look twisted or pulled towards the inner or outer leg it's most likely a fork issue (sometimes combined with strong thighs).
If the tightness is across the seat (tension on the CB seat seam and most likely gaping pockets) it's a width issue and you need to add ease. Adding fork width (1/2 of what you add to the seat width) will help, too.
BTW, that also works the other way round, when you reduce the seat width. When the seat is tight try the next bigger size trouser. It's often easier and simpler to reduce the waist.
When you measure a person's seat width keep the tape loose enough to be able to move it. Check your charts to see what the ease in the size you try on should be. You need at least 2" for close fitting trousers to keep them comfortable.
FYI, for unpleated and/ or close fitting trouser it makes sense to add/ have a pinch dart in the fronts, as it adds width for the hip bone and avoids gaping pockets.
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