After watching the last episode, I must admit I'm a bit disappointed with the changes the writers have done in regards to army sizes as compared to the book series.
In this episode Yara says that she will take the fastest ship and 50 of the best fighters and move across the narrow sea and the weeping water river into the Dreadford. Assuming that she does this, this voyage would take her through the Lannister, the Tyrrell, the Dorn, the Baratheon and the Arryn fleet, as she would have to sail across most of Westeros. Moreover in episode 7 of the second season Tyrion says that Stannis' fleet is moving from the Stormlands to King's Landing and he would arrive within 5 days- 4 if the wind is to his advantage. I suppose this means Yara's voyage would take over a year at best. Finally her army size is interesting. In the book the Dreadford has a garrison of 700 men. It's highly unlikely that with her 50 warriors (who would be tired after a year at sea) and no siege machines, she would win a battle. The only positive outcome would be if she scaled the walls at night, took Theon and left without too much of a fight.
Which brings us to the siege of Winterfell. According to the TV series Theon had 20 men while the Northerners had 200. Assuming that the men had to sleep sometime, Theon's active garrison would be about 12 men. He could not possible defend a city as large as Winterfell unless the Northerners had no siege equipment and the walls were too large to be scaled. Moreover, I fail to see why the Northerners can field a 20000 strong army in the south (not including the Tully allies) but leave their castles practically undefended.
Another thing that bothers me is the Wildings raiding party. In the TV series Mance sends a force of 20 wildlings plus Tormund, John, Ygritte and Orell. A total of 24 raiders. Six die during the climb, 3 die by the wolves and John Snow leaves the party. That leaves 14 raiders. Snow says that the garrison of Castle Black is a thousand men but Orell calls him a liar, which means that the garrison is about 300-600 men. Again I fail to see how this small raiding group would make a difference.
To sum up, I think the writers have messed up the numbers a little.