Chances are, you compiled your kernel in debug mode. Kernels built in debug mode contain many symbols that are used for debugging, thus greatly increasing the size of the kernel. Note that if you running a FreeBSD 3.0 or later system, there will be little or no performance decrease from running a debug kernel, and it is useful to keep one around in case of a system panic.
However, if you are running low on disk space, or you simply do not want to run a debug kernel, make sure that both of the following are true:
You do not have a line in your kernel configuration file that reads:
You are not running config(8) with the -g option.
Both of the above situations will cause your kernel to be built in debug mode. As long as you make sure you follow the steps above, you can build your kernel normally, and you should notice a fairly large size decrease; most kernels tend to be around 1.5MB to 2MB.