Not at all, it is fair by all means. No matter the number of opponents, at one point, there was a champion, a gold medalist. Whatever. Let's say he had only 10 opponents who were damn strong. But today's champion had 10000 opponents... strangely those were all pretty weak. Does that make the new champion stronger than the previous one? Of course not. Stating otherwise would be very partial. The number of opponents is not an impartial indicator of performance.
Of course max versus sum is better.... but it is still unfair. With a fixed amount of points it's far better. Let's break it down :
Contest max points = 100
Contest 1 : 5 successfull contenders (A, B, C, D, E) : who got : 100, 75, 50, 25, 1
Contest 2 : 26 successfull contenders (A, B, ..., Z) who got 100, 96, 92, ..., 1
You see that the champion gets 100 each time, but the 2nd get more points if there are more opponents than if there are less. So with a MAX system as you thought, it's interesting to try again to maybe raise your max whatever the number of participants.
Though some might say it's unfair that contest 1 and 2 don't award points the same way. They are not really wrong. Maybe a fixed reward would be better such as (example for a 1000 points contest type) :
1st : 1000 points
2nd : 900 points
3rd : 800 points
4th-10th : 700 points
11th-25th : 600 points
26th-50th : 500 points
51th-100th : 400 points
101th-250th : 300 points
251th-500th : 200 points
501th+ : 100 points (remaining sucessfull contenders)
Unsuccessfull contenders : 1 point
I think that would be fine too.