The Italian, who managed England from 2008 to 2012, told fifa.com that the team had under-performed in competitions such as the European championships and World Cup "because they're tired".
"They're the least fresh of any sides because their league doesn't have a break," he was quoted as saying. "It's like when you're driving a car: if you stop halfway to put fuel in then you'll definitely get where you want to go but if you don't then there's always the chance you'll be running on empty before you reach your goal.
"In my opinion the football played in the first half of the English season is much better than in the second half. And because of that, if you want to be a competitive team in the Premier League, you need a really big squad, which is a luxury you don't get with the national team."
Capello left the England job to take up a new role as national coach of Russia, which hosts the World Cup for the first time in 2018. Russia are also on track to qualify for the 2014 World Cup in Brazil and lie just two points behind Portugal in Group F but with two games in hand.
But he nevertheless said he thought his time in charge of the 1966 World Cup winners had been positive, with the introduction of young players such as Danny Welbeck, Jack Wilshere, Phil Jones, Ashley Young, James Milner and Joe Hart.
"I felt I left a good legacy," he added. Capello's comments come amid renewed discussion of England's performance at international level in recent decades, with criticism levelled at clubs and league for ignoring the national side.
Current boss Roy Hodgson was reportedly furious at the Premier League for scheduling big matches the weekend before England play key World Cup qualifying games against Moldova and Ukraine in September.