Explanation:
the perimeter/circumference of a square is simply 4 times one side length, because all side lengths are the same.
we could also say
side + side + side + side
and that is simply 4×side.
as the perimeter (= length of the fence) is calculated
as 4×side, any change factor of the side length goes 1:1 into the perimeter calculation.
let's say the side length changes by a factor f
side new = side old × f
perimeter old = 4 × side old
perimeter new = 4 × side new = 4 × side old × f
so, if the side length changes by the factor f, then also the perimeter changes by the factor f.