You might find the 20L simply too deep to fit as the lower part of it could drag on your front wheel - especially if you have front suspension/are riding a 29er.
I can certainly fit a summer bag, mat and bivi in a 13L bag.
DSC_0143 by Colin Cadden, on Flickr
P1010496 by Colin Cadden, on Flickr
IMHO, a 20L bag would only make sense if it was the same overall diameter of the 13L but just longer