In the latter, the /w/ is shorter.
Nope, the difference is that in the first case, we are dealing with a co-articulartion
. With [kw], the labial component (i.e. [w]) begins only after
the velar stop has been produced. With [kʷ], the labialisation[*] begins at or before the moment when the stop is released. Acoustically, this results in a lower second formant
on the following vowel--similar but less pronounced than the effect produced by a labial stop.
(It's possible that the reason Mr Spaghetti does not hear a difference in the stop components is that he labialises [k] in [kw], effectively producing [kʷw].)
[*] Generally lip-rounding, though the exact method of articulation varies by language. See http://en.wikipedia.org/wiki/Labialization
for a brief discussion of some of the possibilities.