You don't need Box here because Vec<T> is already a fixed size handle to heap data.

A vector of boxes is beneficial if you need to move objects around. If each T is 1000B or something, you really don’t want to copy or even do true moves in memory.

Hell, even if you’re not moving things around explicitly, don’t forget that vectors resize themselves. If you use Box, then these resizes will be less painful with large objects.