What most people will tell you is that 7/2s are no longer meta and that you should build 14/4s or tanks with 10/0 to hold the line. And they're right, but many don't totally understand why, and it's worth knowing.
In a combat, if a division targets another with more defense/breakthrough than it has attacks, its attacks will only have a 10% chance each of doing damage. However, if they have more attacks than the target's defense/breakthrough, each "attack over def/bkt" will have a 40% chance of hitting. For example - division A has 100 attacks, and targets division B which has 50 defense. 50 of division A's attacks will be under defense, and only 50*.1 = 5 of them would be expected to hit. However the other 50 would be over defense, and 50*.4 = 20 of them would be expected to hit. Effectively, attacks over breakthrough and defense are 4 times as strong. Usually they're called "crits."
Crits are the reason why generally, larger divisions are favored, especially for attackers. By having lots of attack concentrated in one division, its target will also be more likely to have lots of attack concentrated against it, and be more likely to take crits. The same goes for concentrating defense/breakthrough. However, this comes at several costs - for one, if you only have 2 big divisions in a combat as opposed to 4 small ones, your enemies' attacks will concentrate against them more often and be more likely to crit. Another cost is organization - a 7/2 and 14/4 have the same org, meaning that on the basises of cost and combat width large divisions have far less org. Because of that for defending smaller units are generally considered better. Yes, they take more crits, but defense is cheaper than attack and more org is worth it.
So. How does that relate to 7/2s? Well, many patches back artillery used be much stronger - it had 50% more soft attack than now. Additionally, superior firepower gave 10% more soft attack than it does today. That meant that 7/2s and 14/4s actually had more attacks than 10/0s and 20/0s had defense, and could get crits without having their attacks stack.
When both 20w and 40w crit or don't crit, then the 20w are actually better, since they have more org. As Corpsefool said I made a post showing that when neither crit alone, 7/2s are better than 14/4s and will win battles the 14/4s can't. Another user (on the reddit) made a post showing the same, more or less, with tanks.
There is actually still a use for 7/2s over 14/4s, then, even though tanks are preferable to both. 14/4s do get the ability to crit eventually, but you need to have somewhat of a tech advantage and/or lots of attack modifiers. Against the AI they are especially effective since the AI likes to make lots of low-defense, small and spammy templates.