Why Bandwidth is set mainly by the symbol (baud) rate, not by how many bits each symbol carries. If you use a more efficient code, for example a higher-order modulation that packs more bits into each symbol, or a source code that wastes fewer bits, you move more data per second at the same symbol rate and therefore the same occupied bandwidth. Constellation diagrams show this directly: going from 4 points to 16 points doubles bits per symbol with no change in symbol rate.
Watch out Forward error correction is tempting, but FEC adds redundant check bits, so at a fixed symbol rate it lowers the net user data rate; it buys reliability, not throughput. Analog-to-digital resolution affects how finely an analog source is digitized, which actually creates more bits to send, not a faster channel.
Bandwidth follows baud, not bits. Pack more bits per symbol with a better code and the bandwidth stays put.