There are two reasons. One, a three-member riding requires 25% of the vote to get elected. A seven-member riding requires 12.5%. There's a marked difference in those two. If you plotted this out, it would be a long curve that goes out like this, so I don't think three-member ridings is the right number in that sense.
The other reason is that the study by Carey and Hix indicates that one- to three-member ridings don't do as well in terms of economic performance, in terms of government spending and stuff like that. Their sweet spot, they called it the “proportional sweet spot”, was four to eight members. That's the other reason I chose five to seven members.