For something very heavy, it’s tiny (6)
I believe the answer is:
proton
'it's tiny' is the definition.
(type of subatomic particle)
'for something very heavy' is the wordplay.
'for' becomes 'pro' (as in being 'pro' something).
'something very heavy' becomes 'ton' (unit of weight).
'pro'+'ton'='PROTON'
(Other definitions for proton that I've seen before include "One is positive" , "Stable subatomic particle" , "Atomic constituent" , "Positive nucleon" , "Positive-charged particle" .)