Well it depends on the meaning of "small".
Pennsy engines do tend to be smaller for the same wheel arrangements as many western engines. The Mikado and mountain are certainly not the biggest of the their types. Western Maryland had a bigger decapod, but just a few.
However the have been many smaller PRR engines done in 2 rail.
First MAX Grey imported the D16, E6, and G5. I think this is the only time a D16 has been done in Brass.
USHobbies reran the E6 at least once.
Sunset has done B28 (usra 0-6-0) in a pennsy version, An E6 twice, a G5, an H8/9, and really nice H6sb. I have 2 of the h6s and they are great detail and great runners.
Saginaw and then CLW made a kit for the H10. The CLW kit has really nice lost wax detail castings.
Weaver imported G5 and H10. These do tend to have some mods to also handle 3 rail curves and wheels.
MTH has done an H3 and I think an early D-class. Not sure if these were done in both 2 and 4 rail or just 3 rail. I have seen the H3 and it is a nice model.
and of course from this thread, GEM imported the F3 mogul and the B6sb 0-6-0
PSC I believe has also done a superdetailed B6.
I sort of think a 2-8-0 or smaller is a small loco, but even if you want turn of the century stuff there are still 3-4 out there.
The brass stuff shows up at O Scale shows quite regularly with the exception of the D16. I think I have seen 3-4 at least of these models at nearly every show. The D16 does not show up very often. One was on Ebay earlier in the year and a forum member bought it.