Why are two different length antenna sold with this 433MHz module?
The smaller antenna for transmit is probably a result of trying to match impedance to the transmitter circuit and reducing VSWR.  Why not just use 1/4 wave?  I'm not sure.  It looks like the antennas are pretty close to a 1/3-wave (~ 23cm vs 25cm) for the transmitter and 1/2-wave (~ 35cm vs 32cm) for the receiver, though. Not exact, but close.  The actual variance from 1/2 and 1/3 could be a result of the designer tuning the antenna to work best with those circuits.  Then again, it could just be that they got a really good deal on those antennas and deemed them "close enough" for the application.  I can't look at your link because it's blocked at my work for some reason.