The problem is ownership.

The boss owns the machines that increased the production so they are/feel entitled to all of that increase of profit. "You" didn't do anything more so why do you deserve more especially if you're actually doing less now?

I can understand this line of thinking, I kinda hate it, but I can totally see it even from a moral perspective. Yes if you dig deep enough you could argue that their exploiting of your labor afforded them the ability to buy the new machines, but we also agreed to be exploited by agreeing to work at the place to begin with.

I'm not one of these temporarily embarrassed millionaires that sees themselves as that boss, we just need to think of this in terms of what opposition we'll face if this argument is pushed.