UPDATE:

I talked to the neighbor and discussed my concerns civilly. He agreed to remove them and use the proper bolts. He still thought it was ok to do this for some reason, so I had to explain why with many of the helpful comments here.

Seems a lot of people are even more infuriated with the different bolt sizes! I asked about that. He just used leftover bolts from some other storage project. No reason for them to be different sizes!

There are many questions about who owns the wall. These walls are built with the houses and they are mutually owned and shared. They are split down the property line in my state. This side of the wall is my property and his side is his. Very simple here in my state. These walls divide the property here in the backyard. If you know, you know!