In the example above, the speaker is OK with small meals, but this meal is too small. The phrase "I'm all for __, but ___" is good for talking about which things you're OK with, and which things you're not OK with.
Here are some other examples:
I'm all for saving, but sometimes you have to spend money to make money.
I'm all for helping out the poor, but I don't see why we should be forced to support people who are too lazy to work with our tax dollars.
"I'm all for ___" means that you support something. However, people don't usually say "I'm all for ___" unless they're going to follow it with a complaint.