Big differences:
First, broadcast TV is mostly received by cable, so coverage areas are much larger than over the air coverage.
Second, radio markets are defined by listening patterns. Only the counties where a certain market’s stations are in the majority in listening are part of the market. Radio markets can be adjusted annually.
Third, there are far more radio markets than TV markets, as there are radio station in nearly every town and village in the US. TV stations are mostly in the larger cities, with coverage of lots of surrounding areas.
Radio is local, based on over the air signals. TV is regional, distributed by cable and satellite providers to broader regions.