Many developing singers encounter difficulty as their vowels begin to take proper shape. The sheer power and loudness of some of the pure sounds makes them hold back artificially, causing tension and distortion of the sound.
The truth is that some sounds are simply louder than others. In other words for the same amount of perceived effort on the part of the singer, the various vowels will each generate a sound which is relatively louder or less loud than other vowels.
This is partly due to the way we hear sound, and partly due to the way the sound is produced. Well the fact is that it is a good thing that sounds vary in power. That fact is a major contributor to making singing sound great: it adds texture and contour to the vocal line.
A simple example is to move from oo to ee: you will feel the strong high frequency energy kick in as you form the third resonator at the front of the mouth and the sound will seem much louder.
Don't let the loudness make you back off or 'break' the sound... it's louder than oo, that's just it's nature. Explore the relative power of the vowels, it's a very important tool for styling.