Generally a pleyer is considered pot comitted when his stack is below half the pot.

In practice, let's say you and your opponent have 10$, the pot is 6$.

If I bet 7$ and my opponent calls, i will have 3$ left on the river in a 20$ pot, I obviously commit myself to the pot by betting that big (one way to see it is that if I'm pushed allin by my opponent, I will have 3$ to pay to win a pot of 23$, 23:3 = 7.5:1 odds, you can't fold anything but a pure bluff here.

If I bet 3$ in it, I'll have 7$ left in a 12$ pot. My odds by getting pushed allin would be 12+7:7 = 2.3:1, which are much worse odds, there are still a lot of hands that I can fold if my opponent represents force.

But it of course depends on your hand's equity. If you have a 40% equity holding, you will be comitted to the pot way more easily than with a 5% equity hand.

As usual, the answer is: it depends