Previous estimates put the asteroid's average diameter at 270 metres (877 feet) representing a mass that would equal the energy release of a 506-megatonne bomb, according to NASA figures.
The European Space Agency (ESA) said its Herschel telescope had scanned the space rock as it headed towards its closest fly-by with the planet in years on Wednesday.
The object is being tracked by astronomers to help them fine tune the 2029 and 2036 risks of a collision.
Herschel, using thermal sensors, also found that Apophis is darker than previously thought, the ESA added.
Only 23% of light that falls on it is reflected, and the rest is absorbed by the asteroid. Previous estimates of this reflectivity, known as albedo, were put at around 33%.
This discovery is important because asteroids experience something called the Yarkovsky effect, or an increase in thrust that comes from alternate heating and cooling as the rock slowly turns in space.
Over time, this momentum can change the body's trajectory as it moves through the Solar System.
On February 15, a 57-metre (185-feet) asteroid, 2012 DA14, will skim the planet at just 34,500km (21,600 miles), making the narrowest approach so far of any detected asteroid.
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