I'm sure that small deployments are generally much more successful... but is that because they are intrinsically better or because they are typically used under circumstances much more conducive to success, such as when the government being assisted has a relatively high capacity of its own?

Larger military operations are typically used in cases of full or imminent state failure or in a post-regime change situation, where we are less assisting a state than trying to create one. Those situations would naturally have a lower success rate, but is that because the operations are large or because the underlying conditions are far less conducive to success?

The medicine that isn't used until the patient is in critical condition is likely to have a lower success rate. That doesn't mean it's bad medicine, it means that patients in critical condition are harder to cure.