Fairly hard to answer and probably requires a play with bike fit - a visit to a bike fitter could be worth considering.
Geometry and size - some bikes are more likely to force you into a position that loads the arms than others. You can look to address this though adjustments such as bar height, seat position, stem length. Swept bars can also help reduce the effective cockpit length. Note the current MTB trend of super wide bars effectively lengthens the cockpit. Narrow bars might help - you can trial this by moving the grips (if the style allows it) and controls inboard before cutting bars down.
Riding style - When you ride are your shoulders and arms relaxed and guiding the bike, or tense and controlling it. Is your position correct (attack position, bent arms, torso level), allowing the bike to freely move around under you, or is it upright and tense, forcing you to move with the bike.
Fitness - whats you upper body stamina like? Hows you core strength? If you riding style is perfect, you can do a lot of riding without relying on brute strength, but few riders (I do not include myself in that group) have perfect style and a bit of fitness helps a lot.
One thing that will sap arm strength is gripping the bars - either holding on tight or brakes - poor brakes and over use of brakes. Either way the hands need to be relaxed on the bars, holding on enough to guide the bike and keep the rider connected (both physically and mentally) to the bike. Braking should be gentle and controlled (Ideally with one finger), not jabbing them on with the whole hand. If you holding too tight, it saps energy and you loose feel for the bike on the trail. Think of it as a soft touch in partnership with the bike rather than a firm hand fighting and controlling the bike.