There is no fix for this.
You are trying to reward extra investment in your game.
Whatever form that reward takes (XP, bennies, etc.), by definition you want those who express extra investment to get them. (Although the other answer about giving it to everyone is a cool approach.)
Extra investment requires 1. time/effort and 2. caring enough to do it.
Therefore someone needs both 1 and 2 to invest extra. And the problem is, both of those are mixed together. When someone says "I don't have time," what they mean is "I have the same number of hours in a week, I just choose to prioritize other activities."
Don't believe that? In my group, we have three 40++ hour fully employed tech types, one lightly employed, one lightly self employed, and one unemployed. The ones who write the session summaries (and we do 10+ page PDF ones, not mini-summaries) are the three heavily employed folks. The unemployed guy has never done more than show up to play. (Our GM doesn't give out any XP or rewards for doing it, either.) "Not having time" is always a polite excuse for "I prioritize other things instead." That's not bad; your game isn't necessarily more important than watching Sharknado again or puttering in the garden, but it is what it is. I edit and PDF all the summaries, run our campaign blog, work for a fast moving just-IPO'ed startup, run another tech blog, run two local tech user groups, am a single dad, go to church, mow my lawn, and mod this SE, among other things. "No time" is the most common and most untrue excuse people give in this world. So worrying about "bias towards those who have more time" is a red herring.
So you can stop doing it, or if the problem is specific to them over-leveling from the XP, choosing a different reward or lessen the amount. But in the end, you are either rewarding people for additional investment in your game or not, and if you are, then some people will be more equipped and willing to put forward that investment.