To answer your question, there is no simple ratio as to how much you should be lifting. It all depends on your goals, level of fitness, and what you are trying to acheive.

For example, if you are trying just to maintain or tone up, then any weight you can handle 12 to 15 reps comfortably is fine.

If you are aiming for size consider the following as what to aim for. Yu should still attempt 8 to 10 reps. Although below is a just general info. It all varies with every individual depending not only on fitness level, but also genetic make up such as bone mass. Normally, heavy boned individuals can usually lift more. If your goal is size, concentrate most on legs and back as they are your 2 largest muscle groups.

For Bench Press - Aim for your body weight
For squats - Aim for 1.5 times to twice your body weight.
For Leg Press - Max it out at 1.5 times your body weight
For Bicep Curls - aim for 1/5 to 1/4 of your body weight

I hope that helps a bit. Anything else you need to know please feel free to ask.

there is no exact ratio, but the weight used is comparatively proportional to the size of the muscle. quad's are the largest muscle group, therefore capable of greatest amount of weight used per exercise. biceps are the smallest, resulting in a lesser amount of weight used per exercise. why would you assume equal amounts of weight are used for reciprocating muscles when you know they are different sizes?

I don't think so. I think it comes down to your own fitness level and how much you can put up max per muscle part. Ex. My leg extension is more than twice the amount of my leg curl.