When playing poker, top pair on the flop is not enough to win the hand. For a start you must bet to protect it. At this stage in no-limit hold ’em it’s the accepted philosophy to bet around the size of the pot. This offers any opponent odds of 2-1. If he is drawing to a flush, he has a worse than 4-1 chance of hitting it on the turn. This is a good deal for you, ignoring the added complexities of later rounds of betting and cards. The bet also prices out opponents who are drawing to a flush with an overcard, when they have a 3-1 chance of making the flush or a pair better than yours.
If the flop has cards of three different suits, no flush is yet possible. Now an open-ended straight is the only danger and you don't need to commit so many chips. A bet of half the pot offers an opponent 3-1 against his slightly better than 5-1 chance of completing the straight. Again, it also prices out a hand drawing with an overcard.
Why would you want to bet less? Well, every bet is risky. You may be facing a hand with two pair or a set, both very hard to see but your opponent will probably raise you. Now you have to judge how full of shit he is. Does he really have the goods? Remember: you only have top pair, a hand unlikely to improve, and the default course must be to let it and your chips go. So, it’s as well to be efficient with bet sizing. Over the course of time it all adds up.
Note that a flop with two suited cards matching one of the suits in your hand reduces the chance of an opponent completing a possible flush. In fact it too becomes a slightly better than 5-1 chance and you can bet half the pot. The great thing here is that you know it and your opponent doesn’t! Now your efficient bet sizing can suck him in to a gamble that is not profitable for him and this translates into profits for you.
Incidentally, betting more than the pot becomes a game of diminishing returns. You can never put enough money in to offer evens and betting twice the pot only offers 3-2. If an opponent has a hand strong enough to call 2-1, 3-2 isn't going to make a huge difference, especially when the implied odds of future betting rounds swamp this small margin.