Answer:
The cyclops is compared to a mast because of his size.
Step-by-step explanation:
"...but Cyclops went on filling up his belly with manflesh...then lay down like a mast among his sheep."
A mast is a giant shaft, usually on a ship that carries the sail. It is large to withstand the stress of the wind against the sail. The cyclops is also large and he fills up on the flesh of men making him larger. Laying amongst the sheep implies that he is quite full, and going for rest.
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