Under increasing returns to a variable factor only one factor is variable and others are fixed for increasing output more and more units of variable factor are employed.
As a result of using more units of variable factor, total product increases at increasing rate i.e. marginal product increase. On the contrary, under increasing returns to scale all the factors are treated as variable and increased in the same proportion, as a result of which proportionate increase in the total product is more than the proportionate increase in all the factors.
For example, by doubling of the factors output is more than doubled.