Measuring MongoDB I/O and CPU performance
There are several tools that measure system performance like newrelic,MMS,cacti, ganglia...

Regarding the size you need to share a sample record from Oracle and a sample document from Mongo. 

The obvious reasons are:


long field names: A collection in Mongo with N documents stores the field names N times. If the field names are too long you are wasting space.
powerof2 allocation strategy (only for MMAP engine): Is now the default record allocation strategy for MMAPv1. With the power of 2 sizes allocation strategy, each record has a size in bytes that is a power of 2 (e.g. 32, 64, 128, 256, 512 ... 2MB). If your document size is 33 then mongo will allocate 64 and you are wasting half your storage.
Compression (only for MMAP engine): Oracle is using compression while MMAP engine not