Hi, I dont know all that much about transponders. I would guess that it is better for a transponder to require less bandwidth to perform its receive/transmit. Hence for a given frequency sprectrum, there may be a greater number of transponder channels present.
In general, designing systems that need to operate over a wider bandwidth is more difficult. The design now needs to work accross a greater range of frequencues whereas with a narrower bandwidth, the specifications need to be met accross a more limited bandwidth.
You should first study a textbook on satellite communication to get the basic ideas on how transponders are designed, and why .
Historically, first there were "bent-pipe" transponders, with one 27-30 MHz bandwidth to transmit by frequency modulation. as amplifiers, TWTs were used, with a high gain but a limited power output. To reduce intermodulation and distortion mostly if multichannel telephony was transmitted, the output power had to be reduced by "back-off".
Later, multi-channel transponders were designed, also for FM, and using switches, transponder life was extended by switching to spare TWTs after some died.
Latest, instead of "bent-pipe", signal processing transponders are used, with QPSK and digital multiplexes, spot-beams, etc. Again the new transponders often do not cover a huge territory with a single antenna but they can move the spot-beams rapidly, so certain signal blocks are sent to a defined location, other blocks somewhere else.
Satellite communication is not so easy a "radio" but a very complex system, gradually growing in complexity.
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To add the explanation to what is a "transponder": In satellite communication, you send a limited RF power over 35800 km to and from a geostationary orbit. Uplink, say at 6 GHz, sends up to several kW to a satellite but there it is received at ~ -90 dBm. You cannot transmit back the downlink on the same frequency; this is why here 4 GHz band is used, to prevent oscillation and crosstalk. Using so different receive and transmit frequencies allows to use good filters for isolation. The satellite transmitter transmits back at 4 GHz with 20-40 W, or, +33...+43 dBm, so isolation must be > 130 dB. Similarly at 14/11 and 18/12 GHz, etc.
The device aboard a satellite transponds the spectrum from 6 GHz band to 4 GHz band, so it is named a transponder.