Which one is correct?
His head size is larger in proportion to his body.
His head size is bigger in proportion to his body.

Question

Answer

There is still an issue with the verb is with in.  This would be better:

His head is to big for his body.  (best)
 In proportion to his body, his head is to big. (good)
In proportion to his body, his head is the bigger / larger.  (OK - not as good)
His head is bigger, in proportion to his body.  (OK - Notice the comma)

Answer

second answer l think that .