Each of us has a different financial status. When we say gamble only what you can afford to lose, it means the amount that won’t cause financial problems even if you lose it, regardless of how much it is. Sure, losing, whether it’s a small or big amount will still cause some frustration and that's normal. But at least it’s not to the point where it affects your finances overall. Most importantly, it shouldn’t impact your day to day life.
I think money that you can afford to lose is the extra money you have that can't affect your finances, but if it's a big amount, I am sure if you lose that, it's more frustrating than if you lose a small amount.
There's a big difference between losing big and small; the effect is higher when losing a big amount. Even if you can afford to lose it, I am sure it hurts when you lose, and the effect if you win makes you very happy.