Sharing half of a small lunch means more than donating from a big one.

Generosity gets measured against what you have, not against the size of what you hand over. Giving out of scarcity carries real sacrifice and real risk, and whoever receives it feels that difference without doing any arithmetic. Giving out of plenty is still good and simply costs less. The scale is proportional, which is why small gifts can weigh more.

Notice what you give when you do not have much, and stop discounting it because the amount is small. When you do have more, work out what the same proportion would be. Then give that.