There is a slight advantage of line over mob - a unit in line can move backwards as an action.
In melee, all figures in a unit fight. Page 15 - "Do not be concerned how many figures actually make physical contact with the enemy as ALL the figures in a unit are counted in melee."
If a 10 figure unit enters melee against a 4 figure unit, all 10 figures are considered to be in melee. However, when deciding the number of d6 to roll during a round of melee combat, the number of d6 can not exceed twice the number of figures in the smaller unit.
For an example, take a 10 figure Aletengard Infantry unit with REP 4 & AC 4 in melee with a 5 figure Brethren Heavy Spear unit, also REP 4 & AC4. Both units are on flat ground.
Before beginning the round of melee, each side removes 1 figure. The Altengard unit now has 9 figures & the Brethren 4 figures.
You next calculate the Kill Factor. In this case, both units have a kill factor of 2.
Now you determine the number of dice to roll. The Brethren unit gets 4d6 for its REP & 4d6 for the number of figures in the unit. The Altengard unit gets 4d6 for its REP, but only 8d6 for the number of figures, because the d6s for figures can never exceed twice the number of the opposing unit. So the Altengard player rolls 12d6 looking for 1s & 2s while the Bretren player rolls only 8d6.
Even though you didn't get to roll a d6 for the 9th figure in the Altengard unit, having the additional figure comes into play if there is a second round of melee & each side again removes 1 figure for the melee. The extra figures give the unit staying power through the rounds of melee.