Hugo Kornelis explains the notion of memory fractions:
Some time ago a reader reached out to me with a request for help. He showed me a query and accompanying execution plan, and asked if I could help reduce (or, better yet, eliminate) the many hash spills that were killing his performance.
While helping him work through the plan, I was once more reminded of one of my pet peeves with execution plans: we get to see the requested memory for the plan (the Memory Grant and MemoryGrantInfo properties), which is of course based on the estimated total memory usage of operators that are active at the same time. We also get to see the actual memory used by each individual operator (in the Memory Usage property). But there is no way to see how much memory the optimizer estimates for each individual operator.
Read on for a detailed explanation.