Microfilled composites cannot have a high filler load due to:
Small size of filler particles which decreases  the overall matrix
More surface area per unit volume
Low modulus of elasticity
Wear of opposing tooth structure caused by microfillers
Filler content of microfilled composite  is 50-60% by weight. Because of the greater surface area per unit volume of these microfine particles they cannot be heavily filled.
Reference- Sturdevant 6th ed P:218