[CentOS] OT: Newest kernels much smaller in size

I'm updating my Desktop and curious as to the reason for the huge
difference in size, between the oldest kernel, which will be removed,
and the newest one that will be installed. Question: Did they remove a
lot of things from the kernel or rewrite it much more efficiently?
TIA.

Installing:
 kernel                 i686     2.6.18-128.4.1.el5         updates        15 M

Removing:
 kernel                 i686     2.6.18-128.1.6.el5         installed      40 M