Well if you're a second a lap faster at Spa, given no other variables you'll be 45 seconds up the road by the end of the race.
If you're a second a lap faster at Monaco, again given no other variables you'll be 78 seconds up the road and nearly lapping the other car.
Hmmm not sure if I worded it badly or if you're just not thinking about this properly!
With the exception of Monaco, all Grand Prix events are aimed to have a similar race distance to each other. This is why Spa has 40 odd laps, and Germany today had 60 odd.
Lap length is only relevant when discussing the actual laptime differential between two cars. For example, if Nico was 2 secs per lap quicker than Bottas around Spa and Germany - the Germany pace would be more impressive because the lap is shorter, hence less opportunity to make the gap.
Saying that a car is 20sec ahead at the end of this GP and commenting on the lap length is pretty much the point I'm getting at, the lap length doesn't make the 20sec gap any more, or less impressive than it would if he had a 20sec lead at the end of Spa.