why InGaAs photodiode always small than silicon photodiode?

Hi all

Does anybody know why InGaAs photodiode is always small than silicon photodiode?

For example, G8941 from hamamatsu, it has diameter of 1mm, and 90pF capacitance. and cut-off frenquency is 35MHz.

while for a silicon photodiode, for example, s5971, diameter is 1.2mm, 3pF capacitance, and cut-off frenqucy is 100Mhz.

So, almost the same size, but a huge difference in Capacitance.

is it because the InGaAs has a large "permittivity" than silicon? I searched, but I could not find the the value of permittivity for InGaAs.