in a cold.draughty loft, cold air is liable to blow down the gap between the pipe and the insulation. Far better to use the correct size.
You might find your bigger size fits the overflow pipes, it is worth insulating these so that they do not freeze if you get an overflow in a cold spell.
BTW pipes in an unheated space like a loft or under the ground floor, need insulating more than any others. You should use the Regulation rigid foam lagging, which is as thick as your wrist.
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