The issue is how small do you mean...For 20 guests you can do two tiers, for 30-40 you can still do three, but they'll be smaller in diameter.
This cake was a smaller one. Height-wise it was the same as any other three-tiered cake, but the diameters of the tiers were 4"-6"-8". This size, not including the top tier (in case you want to keep that for yourself) will serve 30-36 people.
For 40-50 guests you can do a 5-7-9" combination.
So yes, there are options for smaller weddings. You don't have to settle for a single tier, and you can still have a cake that looks like a wedding cake.