There were quite a few changes in the kernel config between 17.0 and 17.1 proposed kernel, in addition the kernel version. That is why its size changed.
It should not make a difference what CPU a kernel is compiled under, as the kernel config dictates the compiler output, as long as its the same compiler version. It is true that the x86 kernel is built on a 64bit cpu, but done in the tc 32bit environment.