Some people said some things so I will add. Safety, beavers enjoy steams and pond usually smaller than ones sturgeons live in. Past that they have no aquatic predators. Big ponds keep the wolves and cyotes further away.

Some said underwater entrance.

Another is winter rafts. They will construct piles of wood so that fresh twigs and leaves are below the surface before winter. So along with digging for roots and hibernating fish they can enjoy fresh sticks and leaves. A larger pond means more access to submerged food sources, their preference.