Inductors in parallel reduce the equivalent inductance, similar to parallel resistors reducing the equivalent inductance.
Larger vias also have less inductance. So why use more vias over larger vias?
1. A lot of small vias may be easier to place on a board than a large via, especially if you have oddly shaped areas available.
2. Even though a larger via will have less inductance, the inductance reduction is roughly proportional to circumference. This increases linearly with radius, while area increases with the square of radius. So you're better off using a bunch of small vias over an equivalent area large via to reduce the net inductance as much as possible.