Your comp viewer is not set to Auto and you have manualyl set it to Full and the RAM preview correctly obeys this. You are confusing resolution and zoom level, I think.
No., I am not confusing viewer quality, viewer size, and or ram quality for previews.
I have cs5 at home, it renders at quarter, third, half, full, or auto as asked.
At work, it doesn't. It is really weird.
sorry, at home now, same person, different profile
What setting do you have for the Resolution menu in the Composition panel?
What setting do you have for the Resolution menu in the Preview panel?
If you have several comp viewers open, and one or more of them are set
to a specific resolution (i.e. another resolution than "Auto") then
this specific resolution is used in all comp viewers set to "Auto."
This has had me confused, so close all other comp viewers and see if
it works as you expect it to work.