Compression Equalizes Resolution
720P, 1080I, 480P About the Same After Compression
NB This page uses 540I for 1080I, and 240I for 480I
See DTV Bandwidths for the real numbers behind this diagram.
If you want to know what compression feels like, it is that set of blocks that a digital screen breaks up into when your satellite dish or digital cable system fails. So you can see the compression when it is failing. Those large blocks are part of the trick used for squeezing the information down enough to fit in the narrow channel.
What about 1080p? 1080p has so much information that it doesn't make sense to try to broadcast it. It is a DVD format. If you were to try to squeeze it through a broadcast channel, the compression ratio required would be 111:1. There's not much left after that much compression. The advantage of the higher resolution over lesser formats disappears under such compression.