Later in the book the author reviews attempts to count all of the ants in leafcutter colonies and is critical of the method of weighing ants then using "average ant weight" to get an estimate.
If a queen can be 346mg and her minima fungus gardening worker just 0.4mg I think he has a point!
The minima workers do a kind of gardening that is beyond our imagination. They rearrange hyphae, they pluck away bad bacteria and add good, they monitor moisture levels with granular precision.