10Mb is enough for most operations on a successful tiki site. Im my opinion 16 or 32 is perfect and you'll never have trouble.
As you enter data, things like wiki pages needs memory to parse and convert into whats visible and displayed on the screen, longer pages take up more memory. Finding links between pages, takes memory, modules such as last changes, comments, etc all need memory.
Im going to update the TikiFriendlyHosts page re HostRockets limit. Its ok for a personal site requirement, but anything you need to rely on Id prefer to see more.