Why can I say "thank you" when someone does me a small favor, but when someone does me a big favor there is no way I can say "thank you"

Why can I say "thank you" when someone does me a small favor, but when someone does me a big favor there is no way I can say "thank you" I really feel bad when I can't say thank you to someone, but sometimes I say it altough I don't relly need to lol