This would be affected by the pressure of the air in the ball. From the FIFA Laws of the Game, Law 2:
Quote:
The ball is:
• spherical
• made of leather or other suitable material
• of a circumference of not more than 70 cm (28 ins) and not less than 68 cm (27 ins)
• not more than 450 g (16 oz) and not less than 410 g (14 oz) in weight at the start of the match
• of a pressure equal to 0.6 – 1.1 atmosphere (600 – 1,100 g/cm2) at sea level (8.5 lbs/sq in – 15.6 lbs/sq in)
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That's quite a range of acceptable pressures.
Section 6.3.1 agrees on weight and size, but puts pressure at the low end of the FIFA scale. "The BALL weighs between 14 and 16 ounces, has a circumference of 27 to 28 inches, and is inflated to a standard pressure of approximately 9psi." The ball will be quite mushy - although not as mushy as some soccer players like to practice with.