Probably you are right on the simplest design thing. Quite often bigger is not better - you reach a point where adding more engines means you have to add more fuel to power those engines, which takes more engines, which takes more fuel and so on.
If you have a large rocket and you find it underperforms, try taking out a fuel tank and seeing if that helps.
Are you using multiple side boosters of any kind or do you just have a central core with nothing attached? If you are using the former, I can teach you how to do asparagus staging, which dramatically improves performance. Or you can youtube it because a video will be able to show you how to do it better than I can describe it with words.