Why is Mi-35M so huge in size? Why is Mi-35M so huge in size? 

What is the benefit of having an attack helicopter to be so big?

If there is no benefit, why wasn't its size reduced?

Apache is not as huge as this.
Apache is pure attack helicopter with just two crew. Hind is a mixed role attack and light transport helicopter with additional room for 8 troops. Different role, different size.