In simple terms, the drive power input is equal to the drive power output plus the losses within the drive. These losses are usually fairly small, 5% or so depending on the size of the drive and the drive technology used.
Neglecting the losses, the input power is delivered at a certain voltage and current. The output power is delivered at a different voltage and current. The output voltage is reduced linearly as frequency drops, so at 25Hz (30Hz if you are in the US) the output voltage is half of the value it would have at rated speed. If the power input and output are assumed to be approximately equal, the output current must be double that of the input current because the output voltage is halved for this case. Similar logic applies at 25% speed, 75%, and so on.
This explanation neglects losses, power factor, harmonics and a whole lot of other things, but you hopefully get the idea.
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