The outcome of the Battle is determined by the overall Defence/Offence strength of both sides participating in it. In addition to the number of Units, the strength of the army is also determined by each Unit's individual statistics, the Player's relative Sector Defence bonuses, Allied Reinforcements, buffs that directly affect Unit's strength etc. For instance, if the Battle takes place at the Sector with 20000 Defence Bonus, the strength of the defending side will increase by 200%.
The outcome of the Battle also depends on the composition of both armies. If Player 1 attacks with Battlecruisers and Player 2 defends with Interceptors, which are strong against Air Forces, Player's 1 losses will be significantly higher.
This is why it is important to always recon first before sending the attack.
We also encourage you to visit our Official Forum where players share their experience and tactics on how to minimize Unit losses in Battles.
Please follow this link: http://plrm.me/td_gameplay_en