There are a handful of different approaches to this. One idea is that the dialog channel is mainly just for voices, and so doesn't need deep bass extension. The people that take this approach usually have large left and right channel speakers and smaller ones for the dialog channel and surrounds.

Another idea is to have matching speakers for each of the three front channels, left, right and dialog. The surrounds can be made smaller, but the idea here is to have all three zones up front the same. That way, when sounds seem to pass from left to right, they aren't going through a transition zone that sounds "lighter."

I think the former idea - the one that uses the smaller vocal dialog channel - is probably one that is mainly attractive because of space and aesthetics. I'm sure it probably is better to have the center channel match the left and the right. Having a weaker dialog channel is probably like not having one at all, and using a virtual center instead. In fact, it would seem to me that if a dialog channel were made smaller, maybe it's best to configure the system as having a virtual center channel instead.

I suppose what's best for each individual is probably determined a lot by the way the room is setup, how far apart the left and right speakers are placed, and other factors. There are a lot of good ways to do it.

Wayne