As others have stated, the zip file size has no bearing whatsoever on a mod's performance. The OP's point about slow connection speeds however is a valid concern.
If you don't believe that, here is an example of a mod that would only be about 200 bytes zipped and would nevertheless completely kill your performance.
Code:
local f = CreateFrame("Frame")
f:SetScript("OnUpdate", function () for x=1,10000000 do end end)