The teams in The Premier League are much better defensively than the teams in La Liga. Messi would not score as many goals in England.
They aren't. It's the typical English stereotype. Good defense = big, strong, tall defenders with spectacular tackles. The pace of the game in England is extremely high, which leaves tons of spaces to be exploited. While the approach in Spain is far more tactical and technical. The philosophy is not to get the ball as fast as possible forward ("kick and rush") in Spain. The pace is slower, the spaces are much tighter, therefore scoring is much harder. Has Silva not talked about this recently?
Goals conceded: top 5 teams in PL and Spain currently after 11 games:
PL:
1. City: 10
2. ManU: 12
3. Newcastle: 8
4. Chelsea: 15
5. Spurs*: 15
avg. conceded: 12
(*Spurs with only 10 games)
La Liga:
1. Real Madrid: 7
2. Barca: 6
3. Valencia: 9
4. Levante: 9
5. Sevilla: 8
avg. conceded: 7.8
Doesn't look any different if you look at the bottom 5:
PL:
16. Fulham: 15
17. Everton: 15
18. Bolton W.: 27
19. Blackburn R.: 24
20. Wigan A.: 20
avg. conceded: 20.2
La Liga:
16. Saragossa: 22
17. S. Gijon: 16
18. R. Santander: 15
19. Granada: 12
20: Real Sociedad: 18
avg. conceded: 16.6
You can look at it from another perspective as well. City scored not only more goals than Barcelona so far but also collected 6 (!) more points than us and 3 more points than Real Madrid.
Top scorers in PL currently:
Robin van Persie: 11 goals in 11 games
Dzeko: 10 goals in 9 games
Agüero: 9 goals in 10 games
Rooney: 9 in 10 games
Demba Ba: 8 in 10 games
Those guys manage goal ratios around 1. Now imagine what Messi would do then, considering he's 10 times the player than all of them together (except Rooney, he's white Pelé and so on).
The PL gets too much credit due to their top 4 (ManU, City, Chelsea, Arsenal) and a lot of media hype from the English press.