Aren't Bottom Ends All the Same?

[rohmell]

Here's a bottom end for a 49/50cc engine:
Motorized BIKE GAS ENGINE bicycle 49cc 50cc bottom end | eBay

Here's a bottom end for a 66/80cc engine:
Motorized BIKE GAS ENGINE bicycle 66cc 80cc bottom end | eBay

Now, I thought that all the bottom ends were the same, the difference in cc was due to the cylinder, piston and head.

I guess that is not the case, and how come the smaller displacement bottom end is more expensive?