If you consider just the impact phase of a column of spear charging impact foot then you are indeed correct that there is a significantly lower chance that the spear will disrupt at impact. The issue is that you cannot consider just the impact phase in isolation.
I have plugged the numbers into my single round stats calculator and assuming average troops against average troops you get:
Legionaries disrupted 9.88%
Neither side disrupted 56.79%
Spear disrupted 33.33%
This does not take into account the chance of base loses.
The same combat on a 2 base frontage ends up with
Legionaries disrupted 11.87%
Neither side disrupted 45.61%
Spear disrupted 42.52%
There is no argument at all that in this isolated match up the spear are better off if they charge on a narrow frontage.
Where there is an issue is that by advancing in column the spear run the risk of being disrupted by shooting which is anything but insignificant as even a single average dice shooting with a - POA against a column of average troops with a general in command radius has a 9.26% chance of disrupting the spear and if that happens things are really bad for the spear.
As for stopping the expansion consider this:

The yellow BG has moved to a possition where it can shoot and it can block expansion.










