Effect of unit's attack.

5 is the max that can do damage in a single stack, I think, therefore it would have done more damage to send awing of 5 and one of 6 (6 so that 1 doesn't take all the defending damage)

It is called State Based Efficiency (SBE). It is another factor that influences the strength of your troops when more than one unit is present. It is affected by the condition of your troops and THE AMOUNT (NUMBER) OF TROOP UNITS. Most unit stacks with condition of 100% have a SBE of 100% up until a certain number of units (see below). When you attack or defend with more than the maximum number of a certain type, the SBE of each of that unit type falls dramatically. (The gane assumes there are too many of these units too close together, so they get in one another's way.) You may stack many different TYPES of units together, just be careful that you do not exceed the maxmum NUMBER of each TYPE of unit.

i. Aircraft – Interceptor – 5

ii. Aircraft – Naval Bomber - 5

iii. Aircraft – Strategic bomber – 5

iv. Aircraft – Tactical bomber - 5

v. Anti air – 8

vi. Anti air – SP – 6

vii. Anti tank – 8

viii. Artillery - 8

ix. Artillery – SP – 6

x. Armored car – 8

xi. Infantry – Commandos - 8

xii. Infantry – Infantry – 8

xiii. Infantry – Mechanized - 6

xiv. Infantry - militia - 11

xv. Infantry – motorized - 8

xvi. Naval - Battleships – 3

xvii. Naval – Carriers – 3

xviii. Naval – Cruiser – 5

Here is a link to the forum post fully explaining SBDE:

https://forum.callofwar.com/index.php?thread/6551-state-based-damage-efficiency/&pageNo=1

There is also a X factor calculation in every combat cycle.

https://forum.callofwar.com/index.php?thread/6446-the-x-factor/