If you have good control of "mouth volume" so that it controls the switching between registers, you should be able to get significant changes in how loud you play. The problem is that blowing "real hard" tends to induce harmonics that "trip" the note into a higher register.

Beginner instruction books, in fact, tell you to get the upper (second) register/octave by "blowing harder." Although that "sort of" works, the instruction has always been a little irritating to me. If you continue blowing as you pull the whistle away from your lips, the escaping air should have the same pitch as the note you were playing. (It doesn't have to "whistle" to have a pitch.) If you do that, the "oral cavity resonance" is tuned to the same pitch you're fingering on the whistle, and the two resonances, mouth and whistle, work together. In addition to making the pitch more stable, it gives a "kinder, gentler tone."

If you can "double tune" your notes by holding good agreement between the pipe pitch and the oral volume pitch, you can produce a significant variation in loudness, although at best it's nothing like you can get with a reed (or lipped/brass) instrument. Most beginners find they can produce more loudness change in upper registers, simply because they can't "scrunch down" the mouth enough to back up the next harmonic that the whistle "wants" when it's overblown, especially with the smaller whistles. I have some problem with the larger whistles in my collection because the "oral volume" needed for the lower octave exceeds what I can provide (i.e. I don't have a big enough mouth) [no comments please]

John