An elongated flame is observed when the candle is burning in the presence of gravity. This is what we observe in day to day life, isn't it ? A spherical flame is observed in micro-gravity or in absence of effective gravity.
Why does this happen ? ;-\
When the wick of candle starts to burn by the combustion of fuel with oxygen, heat is liberated. This liberated heat warms up the burning gases and combustion products. This hot air, being less dense, rises up and the cold air rushes in to occupy that vacant place. As cold air is heavier, it stays. This form of motion of air is called convection and this is what keeps the flame upright and elongated when the candle is lit in the presence of gravity.
When there is no effective gravity, while there is both hot and cold air, and while they are both more dense and less dense respectively, the absence of a net gravitational force means that buoyant forces do not exist. Hence the shape of flame remains spherical, and radiates heat outwards in all directions.
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