I've found/learned that if i cut the lining about 1/8 of an inch smaller in both directions - say the outside is 15 x 12 - then I would cut the lining 14-7/8 x 11-7/8. This was making bags that the outer layer was quilted cotton and the inner layer was "plain" cotton.
My reasoning/explanation - the outer layer has to "go further" than the inside layer/lining.
However - if using taffeta or satin - something that frays easily - cut it the same size as the outer layer - and I would finish the seams to prevent raveling.