What is the difference in size between PAR30 and PAR38 bulbs?

PAR30 is 3 3/4" and PAR38 is 4 3/4"

So in other words, you probably can't use a PAR38 in place of a PAR30. Here is a list of PAR bulb sizes. PAR-20 (2 1/2”) PAR-20 (2 1/2”) PAR-30 (3 3/4”) – Long Neck PAR-30 (3 3/4”) – Short PAR-36 (41/2”) PAR-38 (4 ¾”) PAR-46 (5 ¾”) PAR-56 (7”) PAR-64 (8”)