Is it just that a BAC is generally larger and artificially constructed? Or are there any other differences?
BACs are a special kind of plasmid, based on E. coli's F-plasmid:
The BAC is based on a plasmid in Escherichia coli that is termed the F (for fertility) plasmid. The F plasmid (or F factor) contains information that makes possible the process called conjugation . In conjugation, two Escherichia coli bacteria can physically connect and an exchange of DNA can occur.
A BAC contains the conjugation promoting genetic information as well as stretch of DNA that is destined for incorporation into the bacterium.
It isn't necessarily larger than a plasmid.