I'm a little removed from where I'd need to be to answer that very precisely, and probably shouldn't anyway.
They're not small- these are large, multitenant fleets handling huge numbers of NAT gateways all at once. The system has several layers, that scale on different dimensions and have different requirements. And those "requirements" can be very weird.
The part that actually moves packets around is probably mostly full-size c5n's, for the bandwidth.