Yes, I've also seen this on one of my 64MB/32MB routers with ~40K domains. The problem I found is not with average memory usage, but transient usage. Using many adblock domains bloats dnsmasq and, since dnsmasq forks to handle each TCP request, this seems to consume more than usual memory (even with sharing) sometimes. If already low on memory, it's then easy to trigger the OOM reaper e.g. try opkg update. One option is to switch to ipset based blocking e.g. banIP.