the large is good for flannels- minkey- any thicker-heavier fabric-
the smaller one is good for curves, circles, small applique shapes you are using templates for.
there's one smaller than the 28- i think it's 16- it is the one on my circle cutter.
they each have their place- the basic for quilting cottons is the 45- just a nice size to work with- that can handle the bulk of the jobs they are needed for