I go with Sugar Bear!
The size of an alternator was determined by the allover load expected at worst case. The battery size is also included into that equation.
A small battery capacity with a large alternator and a heavy load, would constantly stress the battery. (Fast Battery discharge, fast alternator re-charge and so on) Ultimately, that battery would not live for very long.
The electrical system has to be balanced and designed to the requirements.

If your 80 Amp alternator allows 80 Amps of current to flow, with full load, I'd be happy, but would suspect the battery to be on it's way out. A 80 Amp alternator goes rarely up to that current (or very brief), since the battery would have to be completely discharged.

One can get an idea if the electrical system has somewhere a problem, if the headlights dim, while switching other consumer in and out. But most likely, this may be related to a corroded ground system.

Normally, the alternator supplies only if the battery drops below a preset voltage. Depending on that battery voltage the alternator would try to charge the battery back to about 13.8 Volt. As the battery voltage increases the current will decrease until the battery reaches the voltage regulator cutoff level. That's when the regulator takes the alternator out of the loop. During driving with some consumer on, this probably happens rarely, cause the battery gets discharged and then re-charged by the alternator, constantly. Without any significant load (consumer) the current maybe very low, perhaps in the range of only 10 Amps. With a 72Ah battery (healthy) it would take about 7 hours to discharge to 11.8 Volts. If my memory doesn't play tricks on me, the regulator will switch the alternator in at 12 Volt (nominal battery voltage) perhaps earlier at 12.6 VDC.
The current and voltage from the alternator is dynamic and one can not say it is good or bad unless the load condition is known.

__________________
Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert