Changing the max block size from 1MB to 0.1MB is a softfork, because it makes previously valid blocks (0.1MB-1MB) invalid.
I think changing the max block size from 1MB to 0.1MB is a hard fork, actually. Miners that don't upgrade will generate one 1MB and those won't be accepted but the upgraded clients, forking the blockchain.
A soft fork would be changing the max block size one
produces from 1MB to 0.1MB, while leaving the max block size one
accepts unchanged.
Or am I wrong? #sincerequestion
Sorry, but you are wrong. Changing max block size to 0.1MB is a soft fork. A soft fork only works if the majority of miners agree, in which case all miners accept the shorter blocks and the minority that produces larger blocks will be ignored. If you do a soft fork only yourself, you are on a short chain and the longest chain will still contain large blocks. It is similar to a hard fork where you are the only one who hasn't upgraded.
Changing the max block size you produce from 1MB to 0.1MB is not a fork at all. You still accept the large blocks of other miners and other miners will accept your block.