Small when compared to the Delta Flyer, which could also be a bomb in a pinch, but large compared to most other things because of the energy expenditure in making something ready to go big boom.
This discussion is interesting--seems they don't actually hit zero torps until Night in season 5. That's not so bad continuity-wise:
http://scifi.stackexchange.com/questions/23850/how-did-voyager-replace-its-photon-torpedoes
Also I don't know if you'd include the ST Tech Manual as acceptable canon--I don't ordinarily count book anything--but it does say you can build torps with planetary materials, which makes sense, because it's that or replication or nothing, and Voyager had plenty of access to planets. While I wouldn't ordinarily count anything off screen as canon, in this case it's one or the other in 24th-century Trek, given the right tools: either you replicate, or you build from raw materials gathered on planets or in space. Torps have to be manufacturable
somehow. So I accept that they built them from planetary stuff based on the logic of process of elimination.