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. Lip rounding is similar to partially covering the mouth. This lowers all the resonant frequencies. In singing, we usually don't cover our mouths with the hand, but we can cover it by rounding the lips. In combination with larynx height adjustments, lip rounding or spreading can be very effective in darkening or brightening the vowels.