A horizontal stretch or shrink happens when you multiply the parent function (in this case f(x) = x²) by a number. If this number is between 0 and 1, the graph will be stretched horizontally. If this number is greater than 1, the graph will be shrunk horizontally. If the number is between -1 and 0, the graph will be stretched horizontally and flipped; if the number is less than -1, the graph will be shrunk horizontally and flipped.
The graphs that are horizontally shrunk are steeper than the others. This is m(x)=4x^2 and g(x) = -4x^2.
