I think the thing that doesn't ever get explained well is the passage of time. They very rarely have lines of dialogue that say something to the effect of "It's been 6 years since I joined the Night's Watch"(for example). I think theres a couple of interviews with George RR Martin where he discusses that he wanted to avoid any definitive statements about the size of Westeros, and he also wanted to avoid any definitive statements about the passage of time between event A and event B, mostly due to how all of us would sit and pick it apart each time it seemed like an army managed to march a distance far faster than any army in history was able.
I think we're to a certain extent supposed to accept that its been years and years since the last major battles on Westeros(any of the battles the Northerners fought against Lannister armies). So its almost certain that a good chunk of young men who were under the cut off line at 12-13 years old during the fight at the Whispering Wood(for example) are now old enough to fight. It seems like its at least a few months to travel across the Narrow Sea so, when Theon and Yara steal all those ships, there is at least 1 year between that event and the Iron Fleet sailing to King's Landing. I would think that the Iron Born being a seafaring people would be able to build a lot of ships very quickly if needed(I think I read about a Belgian/French Shipyard that could pump out a ship of the line size ship every day).
Looking at the time span of wars in the pre-industrial age, could last decades. But it would sometimes take several years for two armies to maneuver around one another to create an advantageous position for themselves before they would decide to engage, and even then, if the battle went sour, they would quickly retreat. Very very very rarely would two pre-industrial armies engage another force when it was not a completely pitched battle(or they had a major trick up their sleeves like a secret force on the enemy's flank).
My guess would be the Iron Born spent a lot of time raiding, and retreating. They would probably never risk a significant force against an enemy prepared for battle or of similar strength so they probably lost relatively few of their fighting strength men during the course of the War of 5 Kings.
After the Red Wedding the Northern forces ceased to be an effective fighting unit, either they were slaughtered at the Red Wedding itself or melted into the country side avoiding battle or turning to banditry. I'm sure a lot of them scurried home as quickly as possible, but its unlikely the Freys or Lannisters were able to inflict a significant(greater than 50%) number of casualties to Northerners.
Stannis' relatively paltry forces of 8-10 thousand(I might be overestimating) suffered rather few losses during their engagements with the Wildlings so I doubt he lost a significant amount of troops, some may have settled or continued to stay at Castle Black. In the TV show we saw his army decimated due to weather and Bolton forces true, but the Bolton forces suffered few casualties. I'd doubt the forces under Jon Snow's command would have killed the Bolton/Umber/Karstark forces to the man so I'd bet a good deal of their levies would have turned and now fight for Jon Snow.
Dany at this point is the only one on Westeros with a relatively fresh army and the Dornish as well, who will probably declare for Dany now that she has arrived. The Tyrells and Lannisters have relatively large armies still but, the Tyrells will probably turn soon enough, neither has fought a significant engagement where the risked a large number of troops.
So I think with the passage of time, movements of mercenary groups, and the relatively low number of large pitched battle engagements, there is of course been a significant drain on the population in Westeros, but I think they have a larger pool of fighting men than they let on(or far fewer men of fighting age have died than the propaganda spewed by sycophants lead us to believe).
Pitched battles where two armies faced off and fought one another, that the outcome of the battle was, one side completely destroying the other, were relatively rare in the pre-industrial age, ***BUT*** they were by far and away more well documented by contemporary writers than any of the other engagements that occurred. If you were a battle field observer, would you write about the 1 huge battle where the Sycthians were crushed by the Macedonians, or the dozen other engagements where the Macedonians or Sychthians only lost a couple hundred horse/infantry a piece and disengaged from one another(or the hundreds of other times where one side managed to march a little faster and got to a hill before the other side, and the other side decided "nah, we're gonna go home this seasons instead of getting crushed")?