I started a new campaign with Scotland and realised that you dont have to have lower numbers that your opposition to gain a "Famous Battle" (sorry about title). I took my tartan army down to england and engaged an english army with around 200 more men, I cleaned the floor with them only taking around 100 casualites myself, After the battle the crossed swords appeared!

So ive come to conclusion that its not always a heroic battle (ie fewer numbers and a great win) to gain a memorial marker stone, Its just an amazing battle win where you indeed massacre your foes