For me Synapse idles at approximately 7MiB, Kupfer at 23MiB.
That is with nearly all except four plugins off for Kupfer. I thought it was supposed to be the lightest and I love the plugins for it.
I used to be a die hard Gnome-Do fan, Then I switched to Synapse when it lost development because I heard a developer from Do went to Synapse.
I want to like Kupfer more but if the memory consumption could match Synapse I'd be sold.