The binary size changing isn't guaranteed after a patch (..the compiler may produce only slightly different code), it all depends on the extent of the change.
Check the build string near the top of the kernel dmesg, it'll show information about the build.. also an MD5/SHA256 sum will help verify the binary has changed.
Make sure you checked out OPENBSD_4_7 not OPENBSD_4_7_BASE from a CVS tree, there haven't been a lot of changes in stable, and I only see 3 kernel patches on the errata page, 2 of these were single line patches.
Also, after building the kernel, make sure you install it..