Why is SQL not using very much RAM?
Windows has a couple of different ways that apps can allocate memory.
In SQL 2012 Express as well as prior SQL Server "paid for" editions, when SQL needs RAM, it would use the normal way of requesting memory. This makes it show high memory usage in task manager where you, me, and most people are used to looking.
However, in SQL Server 2012 "paid for" (Standard, Enterprise, etc) when SQL needs RAM, it may use an "AWE" allocation. which does not show up in the task manager's process memory usage, however, if you do look at server load (the Performance tab), you will see that a lot more memory is being used than what the process list indicates.
We use RAMMAP to see the "actual" memory usage from SQL since SQL is one of only a few apps that do an AWE allocation.
If you look at RAMMAP and then restart SQL (and refresh RAMMAP), you'll see the AWE allocations free up.
Based the best information that could be found from Microsoft, the reason Microsoft moved to AWE was because "regular" memory can be paged but AWE memory can be locked to actual RAM. Using AWE prevents SQL from asking for a ton of RAM only to have the OS page its RAM back out to disk.
As an Example, here is a screenshot of the AWE allocations before and after a sql restart and a screenshot of task manager: