For 2015-2016, here is the difference between the goals allowed by the club with the fewest goals allowed and the goals allowed by the club with the fourth fewest goals allowed in the best leagues: Premier League: 1 (36 - 35) Bundesliga: 25 (42 - 17) Serie A: 20 (40 - 20) La Liga: 17 (35 - 18) Ligue 1: 19 (38 - 19) Primeira Liga (Portugal): 14 (35 - 21) The Premier League had a tiny number.
In the last 6 years, the best defense in Premier League conceded at least 27 goals (Chelsea 2013/2014), which is quite higher than the average result for the best defense in the other leagues (except France before the supremacy of PSG). In England the defense is not so important to win the title, while in Italy it's almost always true the opposite.
Barcelona and Real Madrid score enough that their defense isn't important. The range of goals scored in 2015-2016 La Liga was 78 and the range of goals allowed was 56. Real Madrid had 90 points while allowing 1 fewer goal than Malaga, who had 48 points. Atletico Madrid finished third while allowing under half a goal a game due to how much Barcelona and Real Madrid score.