There's a difference between the two as they are two completely different things. It would not make sense to compare them or expect that they should be equal.
The Size column in your kextstat output shows much memory the kernel extension itself takes up in memory. This is the amount of the memory that the code/data in the kernel extension itself takes up (i.e. comparable to the binary file size of the kernel extension). It doesn't have anything to do with how much memory the kextstat has used. Kernel extensions can dynamically allocate memory while running, and it doesn't show up in the Size column here, as it is a different thing.
The kernel_task memory usage shown is htop shows the actual memory usage of the kernel - this includes dynamically allocated memory. It can include memory used for the kernel itself, kernel extensions or memory allocated by the kernel on behalf of applications.