The acoustic differences that allow us to distinguish between the various vowel productions are usually explained by a source-filter theory. The 'source' is the sound spectrum created by airflow through the glottis, which varies as the vocal folds vibrate. The 'filter' is the vocal tract itself, the shape of which can be controlled by the vocalist. We perceive vowels on the basis of the two lowest formant frequencies of the vocal tract. If you have not yet done so, it may be helpful to read the previous tutorial describing how vowels are formed .
All formant frequencies decrease uniformly as the length of the vocal tract increases.
For this reason, one uses long organ pipes, long strings, large drums, and big loudspeakers to produce low notes. We cannot change our vocal tracts' lengths greatly, however. This produces a comparable percentage shift in the formant frequencies. All formant frequencies decrease uniformly with lip rounding and increase with lip spreading. In the above exercise, you probably had to round your lips in order to protrude them, because there is only so much tissue to work with.