Well it seems a bit natural.. my 2 male RES also do this. Funny I got them at the same time and one turtle is a lot larger then other. I'm talking 7 inches to maybe 5... doesn't sound like much but hes just overall a lot bigger width, etc..
But the little one is a jerk I would say and likes to bother the big one. My turtles aren't eating much right now - they always act like this come winter even though I don't touch the temperatures.. I suppose they look at the weather outside from the window.
So what I did was get my small RES (I get pissed when he drags on to the feet and my big turtle trys to swim away slapping the water), well I get my small RES and put him in the smaller tank which is mostly used for feeding but has a heater and filter too. I would keep him there for a night or a few hours then put them back in. This didn't seem like it had any effect but now whenever I am around the small RES won't bother the bigger one. I suppose if I am out the house anything goes, but I figure it's just a phase.
This phase is rather annoying so make sure you keep your turtles ok. One time my 2 res got into a big fight, and I didn't see it but I saw the battle wounds. The big RES had skin rip on his forearm and the little one msut have gotten bitten to out of retaliation (the big one rarely retaliates, but if he does, it's harmful!) The small RES had a wound on his foot and one of his nails on his arms was missing. I was alarmed but it grew back months later... it seems certain time periods they don't get along. Like I can vividly remember this year - January through June was fine, no fighting. Come July, fighting daily! Then nothing until October where the daily fighting continued. Now it's November and I rarely see them in fighting stance so I suppose its just nature