The middle C to F range is peculiar because it is the range in which we enhance the sense of “cavita'” – increasing significantly 1st formant strength in the voice. This feels like a spacious, almost hollow feeling in the back of the throat. The question arises of how one ascends over the F into the higher register. Why is it an issue? Because the width of the 3rd layer, this sense of hollow width tends to loosen phonation and separate the chords if you let your attention get monopolized by it.
If your focus and mental intention is dominated by this width in the middle voice, which is a very easy trap to fall into, especially in the theater, it can cause you to progressively increase the breath pressure moving through the chords. What I call the first layer of the vowel: this sense of imbedded head tone in the voice is “too nourished” – too strong; exceeding the balance of correct vocalism.
So, here is the key: it is possible to keep the voice compressed and still have the width – in other words you don’t have to be loose to have width. This happens when width is not the only component in the vowel. When the voice is compressed it also has significant higher harmonic activity.
Perhaps it is important to talk in terms of sensations too. When your voice is compressed correctly and has width, it doesn’t feel stuck in the mouth or throat. This requires you to modify the timbre in this range. You don’t have to be heavy to get width. You can keep the breath pressure balanced, find compression in the chords, thin the chords slightly, find the mask resonance, and get width still. Why? Because width is a sound. It’s only a sound. You can have that sound by arranging the harmonic strength of your voice in a way that doesn’t cause you to exceed the baseline of the pressure that should power your voice.