I cannot get full screen emails. What’s with the the list of received emails on the upper screen.

I am unable to see emails on a full screen.  The new emails take 2/3 top screen and as I open one the lower 1/3 of the screen opens which is not large enough to read the entire email.  I must scroll down to read the entire email.   Why are the recently received emails listed on the top portion of the screen while reading one of those emails?