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.
You have to consider that Synapse uses zeitgeist as it's backend and that also uses several MB of ram, so the comparison is not exactly fair.
Also, as MichaelMS pointed out it's wanted that those programs use memory! You want to access your data/actions/bookmarks really fast so you don't want to access the slow HDD for it. The more ram it uses the faster it will respond. It's not a bug it's a feature ;)
It's a trade-off. Kupfer might have a larger memory footprint (and you're right, on my system it reserves 32 MB of writeable/private memory), but it is fast to load and find the desired application/file. It's up to you what you prefer.
And (come on!) are you running on 128MB RAM or something?
Whenever kupfer offers a zeitgeist plugin I'll drop synapse, until then I'll use both.
Kupfer : noticeably better at delivering/predicting the program i want to launch or the document i intend to find.
Synapse : feel is FAST, thanks to zeitgeist backend.
Hence, I'll use both because they're complementary. Given the apparent similarity between gnome-do, syanpse, kupfer, I assume we'd all appreciate a merger of all these projects.
Keep synapse as a name, it is the coolest one ;-) and as such sticks better to memory.