It's probably the cold winter that keeps the leaves small on the selloum. Mine had huge leaves until it died back this winter. This was the first time it has ever died back and the leaves are just now getting larger, but not nearly as large as they were last year. I never fertilize it, but might try that this fall.

My hydrangea was slow to come back this year too and has stayed small. It may be that we got hot very early this spring and that slowed it down.

Might be too much direct sun.

I replanted similar plants under the dappled shade of an oak tree and the leaves all increased in size, but also, the hard freeze set them back a bit this year early, but they are now coming on larger.

Kt