The EVM works with 256bit/32byte words (debatable design decision). Every operation is based on these base units. If your data is smaller, further operations are needed to downscale from 256 bits to 8 bits, hence why you see increased costs.
Btw, if you toggle the "Details" on the online solidity compiler, it will give you the exact assembly dump where the extra opcodes are from. I didn't have time now to interpret them, but if you do and find there's something extra, I'm sure the Solidity team would be happy to add optimizations to work around them.