We use not as … as to make comparisons between things which aren’t equal:
It’s not as heavy as I thought it would be, actually.
Rory hasn’t grown as tall as Tommy yet.
She’s not singing as loudly as she can.
They didn’t play as well as they usually do.
We can modify not as … as by using not quite as or not nearly as:
The second race was not quite as easy as the first one. (The second race was easy but the first one was easier.)
These new shoes are not nearly as comfortable as my old ones. (My old shoes are a lot more comfortable than these new shoes.)
We can also use not so … as. Not so … as is less common than not as … as:
The cycling was good but not so hard as the cross country skiing we did.