I would say a bigger pond is worth more, probably a linear relationship.

So the current system solves for wins (1.0), draws (0.5) and losses (0.0).

In Ponds for 16 players you could have:

1st 4.000
2nd 1.850
3rd 0.950
4th 0.500
5th 0.270
6th 0.150
7th 0.100
8th 0.060
9th 0.038
10th 0.028
11th 0.020
12th 0.015
13th 0.011
14th 0.009
15th 0.007
16th 0.006

Then you calculate the average rating of all your opponents, and calculate the rating the same way as other games.

Or something like that.