Betting Levels

It seems that whenever I am betting big ($25) I lose during that session and that when I am betting small ($10) I win during that session. I am not worried about dealer cheating because I think that problem is way overdone.

I have read much about bankroll requirements and wonder if these requirements only work for someone who chooses a betting unit level and sticks with it or if it doesn't matter that you change betting unit levels throughout a trip. My thoughts based on my experiences is that one needs to choose a constant betting unit level and stick with it forever. I hope this question makes sense.