You know, it might just be me, but even though it's the hip adductors (or abductors? I'm not sure of the name) that keeps the guard closed, I swear it seems like the bigger the guy's legs are, the harder it is to not only pass his guard, but just to be in it.
When I'm with guys with really strong legs, when I go to pass, I feel really off-balanced. And I wrestle; my balance is pretty good. I'm not one to get swept.
To be honest though, my best advice would be to focus more on your guard game and less on how strong your guard is. Guard passes rarely rely on brute force. They either use trickery (like baiting) or simple mechanical leverage. After all, Jiu-Jitsu is about the smaller guy overcoming the bigger one.
Just do situational sparring. Have a guy start off in your guard. If you can sweep or submit him, you start over with him in your guard again. If he passes your guard, then you start over.
It's more about the skills than the strength, IMO. Just develop your leg muscles and spar more. But if you really want, the situps on a heavybag do work well.
Edit: sorry, I didn't realize you said solo drills. Kind of having an off day today.
Umm... I'd say what I've learned about Jiu-Jitsu (and all effective martial arts for that matter) is that the most important part is your hips. Think about it. Your hips are your center of gravity (the Tan T'ien; the point in some Asian philosophies that your Chi or Ki center is located); your hips and core generate all athletic movement. All effective martial arts, like boxing (twist your hips for punching power), Muay Thai (kicks and knees are driven by the hips; not the legs), Judo (hips must be under your opponents' for a throw to have leverage), wrestling (sprawl by arching your hips), and BJJ (HIP MOVEMENT) realize the necessity of your hips.
Dumbbell swings are an exercise you can do that works your hip muscles. Work your core and hip movement a lot.
Also, by nature, humans move pretty inefficiently on the ground. We're used to standing up. Get good at moving around on the ground. Do lots of shrimps, reverse shrimps, neck bridges, arm bars and triangles in the air or on a grappling dummy (you can make one for 60 dollars), bridges, kip-ups, army crawls; just all sorts of weird things that would appear to the common person as an epileptic seizure. Get your movement on the ground to be explosive, quick, and energy-efficient. Most offensive moves from the guard rely on speed and trickery; you need to focus more on how quickly and easily you can move while on your back.
Also, do lots of flexibility exercises. Yoga is always a good fallback. Some of the best guard players who I roll with are really flexible. This one guy I roll with, always seems to get the grip for an armbar from the guard, no matter how he gets stacked or twisted around.