What was the context?

In NLHE, I wouldn't consider any bet to be "big" unless it was more than the size of the current pot. There are a few reasons to use a bet this large. At the micro-stakes, you might do this against a player who you believe will not fold to ANY bet size, and you believe you have them beat.

In a tournament, you might find yourself making "big" bets, since any reasonably sized (smaller) bet basically leaves you with too few chips to have a reasonable chance at winning the tournament if you lose. This is one situation where you hear people say, "Go big or go home."

In limit poker games, there is often a point in the hand where the betting amount is doubled. These larger bets are called "Big Bets". E.g. if you were playing 7-card stud, then you might expect the bet size to be $1 for the early streets, but $2 for the last couple of streets.