Why is the grounded capacitor smaller in a transmitter vs a receiver?
In both cases a capacitor is needed to ground the Base at RF frequencies. However in the transmitter the Base is also the input for FM modulation, so the capacitor must be small enough to not attenuate the audio signal. 

In the receiver the Base is fed with a DC voltage from the regen pot, so the capacitor can be larger. 10uF might be a bit too large for high frequencies though, as the capacitor's parasitic inductance will start to resonate with its capacitance and make it less effective.  At 15MHz it's probably OK, but at 100MHZ a smaller capacitor might work better.

So one answer is:- because they work on different frequencies.