Period trousers should be a very loose fit. Not like today jeans and slacks.

To check your fit, they should rise to or just above your belly button.

You should also be able to fit your fist between your belly and the trousers.

If yours are like this then they are fitting correctly.

If they are two tight you will just blow out the crotch seam.

Suspenders are intended to hold the trousers in place.