I supposed they moved in small bodies because more men could not be passed over a single road on the same day with their artillery and necessary trains.

Later I found the fallacy of this belief.

The rebellion, which followed as a sequence to the Mexican war, never could have been suppressed if larger bodies of men could not have been moved at the same time than was the custom under Scott and Taylor.
The victories in Mexico were, in every instance, over vastly superior numbers.

There were two reasons for this.

Both General Scott and General Taylor had such armies as are not often got together.