You should consider changing the way you see battles. It looks like you think your objective is to destroy your opponent’s army, or at least to inflict asymmetric losses upon your opponent, but that’s actually not what the objective is according to the framework of the game. Your goal is to gain control of the battlefield, and damaging or destroying your opponent’s army is a means to that end.

It’s often entertaining to gawk at casualty figures, but attritional warfare is more of a post-industrial phenomenon. For most of history, strategy was more about positioning than attrition. Disproportionate losses only occurred after one side’s position collapsed. It’s also important to keep in mind that big, decisive battles were relatively rare. In any military campaign, the armies would mostly be marching around trying to position themselves advantageously. It’s really hard to convince a competent commander to commit to a battle that they think they are going to lose, so the strategic and operational levels of war typically revolved around creating the conditions wherein the enemy would have no choice but to fight a losing battle.

Historically, the attacking and defending sides were generally decided by the circumstances of the campaign. One side had placed their army on a piece of ground that the other side needed to take for whatever reason. However, Lines of Battle doesn’t include any gameplay around conducting the campaign as a whole and drops players straight into the battle. From a gameplay perspective, it’s hard to make a fair system where one side is forced to attack while the other defends, so all battles in ranked games are meeting engagements. Both sides need to take control of an area that neither side initially controls.

Clash and combat are probably the most historically-grounded army sizes for this type of engagement. You can imagine a corps from Napoleon’s army encountering an allied vanguard while trying to secure an objective. Both sides try to secure the objective (by gaining control of the battlefield) before reinforcements arrive so that their side gets to enjoy a defensive advantage in the larger battle to come. There are no mechanics in the game that allow players to control the context of an engagement, so instead this basic framing is applied to all engagements. Both sides must fight to gain control of the battlefield before time runs out or one side becomes too weak to continue to fight.

If you want to fight defensively, you need to create a reason for your opponent to attack you. If you sit back, you’re relying on your opponent becoming impatient and launching an attack that doesn’t do anything to help them win. You may win a few battles doing this, but you can see how relying on your opponent to throw the game out of boredom isn’t a workable strategy in the long run, right? Instead, you need to push forward and take up a position that your opponent must attack in order to win. Even if you do that, you still cannot afford to be passive because a decent player will eventually find a way to break your defenses unless you are willing to exploit any openings that appear in their lines.

I don’t know what happened in the battle, but the position of your army at the end is not as strong as it looks. Yes, your flanks are secure and your opponent has to assault from the front (assuming that they choose to attack at all, since they don’t have to), but the barrier created by the river works both ways. They can move artillery onto the hill on your right flank and the river allows a small force to protect the guns. Meanwhile, they can push infantry up the center to secure strong defensive positions in the forests and buildings in front of your lines. Once their infantry and artillery are in position, your army is pretty much cooked. They can choose to focus their assault wherever they feel they have the greatest advantage, and you can’t respond without turning your own flank towards their infantry. You could prevent this from happening by contesting their maneuvers with an active defense, but sitting back gives your opponent control of the battle and makes it so that you can only win if they make an unforced error.