So what you are saying is it should take less effort to shoot at a low level, as it would to shoot at a more experienced player? (Or throw a punch or what ever attack is the one you like best. I prefer to shoot, so I will use that) Yes, ok a more experienced player is better able to dodge my bullets, Pullo actually throws them back at me, and lower levels are easer to defeat. Your hit points are not just the amount of physical damage you can absorb, but also your ability to avoid damage. This would be a measure of how well you react in combat situations. As a higher level character you have had more exposure to combat then a lower level character and are able to better defend yourself. So it would stand to reason then after a fight with a lower level character even though your hit points show that you have lost say 20 points, your character may not have actually taken any physical damage. It is a representation more of his emotional state after the fight. He is slightly rattled, a little jumpy, and maybe coming down from an adrenaline rush. He is not in top fighting form, but not in too bad shape. A quick snack would calm his nerves, enter the classic beef jerky.
As for energy, it takes no more or less to pull a trigger, swing a sword, or throw a punch on a low level as it does to do same on a higher level. I have always thought, how ever that a character should get a happiness boost when they win a fight, or pull off a high level crime. These things would make the character happy to have accomplished them. On the down side it would only be fair to loose happiness when loosing a fight, or crime. It would be a good way to boost the happiness so you can get back in the gym.
I hope this was helpful for you.