I just submitted two grant applications to NSF. The funding mechanism of NSF is quite different from that of JSPS according to my own experience.
The Investigator-Initiated Research Projects, or the NSF regular grant, is applied by researchers in all ranks. A new comer will have to compete with veterans. While Grant-in-Aid from JSPS provides many categories targeted at applicants with different ranks and experience. People compete with their fellows in similar ranks.
Reviewers are definitely experts in the field for NSF grant applications. While the Grant-in-Aid proposal can be reviewed by people from similar fields.
The amount of fund is fixed in a certain Grant-in-Aid category and 30% of it is for administration use. One should ask for exactly the same amount in the proposal for research even though he knows that only 70% can be used for research. The average amount of a NSF PHYS regular grant is ~$100k/year. But one can ask more than that as long as it is justified.
Activities mentions in this site have been supported by the following grants: