Developed - became bigger (4)
I believe the answer is:
grew
I believe this clue is a double definition.
'developed' is the first definition.
('grow' can be a synonym of 'develop')
'became bigger' is the second definition.
('grow' can be a synonym of 'become bigger')
(Other definitions for grew that I've seen before include "Became larger" , "Increased in size" , "Expanded - flourished" , "Got bigger" , "Rose" .)