All depends where it's published really, but in the UK I could't return a short article when we are assessed. However more people might read the shorter article. They have a different sort of impact.

Thanks Chamberlain. Probably I should have presented the case here only for the high-impact journals, then. I would imagine the readability of papers (of all kinds) is accounted in the calculation of the impact factors.

If by a short communication you mean the conference contribution or something similar, then it is next to obvious that the impact of such a publication will be rather small. Contrary, the letter to editor (if a given journal supports letters) often has quite a high impact.