are you talking about a bet as an absolute value, or as a percent of the pot?
If you are talking about if the villain makes a lower turn bet than flop bet, it can mean he didn't hit the flop but is trying not to give up the betting lead in case he makes his hand on the river. It can also mean he thinks you are super weak and is trying to get some value from you by pricing you into the pot. Really need to depend on your read on this opponent and what he was holding last time he made this move.
Let me reverse the flow of your question. Why do you increase bet on every street? Just as an aside, Betting and bet-sizing is a question of your goal. Are you betting to keep your opponent in the pot, trying to keep the lead to force them to make decisions, trying to get them to fold, or trying to keep control of the pot size? As the previous comments indicate, there are a number of reasons why someone would reduce their bet size. So is it one of the above reasons, or is it because they don't have a clue? That is the tough part. Without knowing your opponent, you have to dismiss his actions and keep playing your style.
Bigger bet as as the pot gets bigger rack bet will be a %. The pot never reduces in value
yeah i lower my bet all the time after making big ones, especially vs good players who tend to read it as weakness. But mainly i'll do that when i want to get value when i know the river has bricked, instead of checking i'll sometimes bet really small to either get them to raise/bluff me or to get A high or some really weak pairs to call.
I was going to say, you can definitely use it as a fishing lure for aggressive players. But yeah, you got to try to rotate your tricks too. People catch on and it helps if you set it up. Play loose and tighten up or visa-versa just switching up your game usually helps keep scents thrown off. Poker is interesting, everyone is fighting for dominance and control and we can all disillusion ourselves into thinking we have that control when we are still dependent on catching the right cards.
Personally, I this technique is used to provoke a bluff or hide the hand in very rare cases, this weakness.