With regards to building muscle and/or strength and exercising there is something that is far more important than the size of the weight that you are lifting. That is not to say that you should not try heavier weights and develop, but there is something more important, in my experience, than that.
What I am talking about is form. Doing the exercises properly is the most important thing to do. Form, is far more important than the size of the weight. Completing exercises properly will allow a person to get far more from their workout than if they rush the exercises, for whatever reason.
You should not attempt to lift a weight that means you cannot complete the full range of motion required for the exercise. If you are unable to lift the weight properly then it is likely that it is far too heavy. Not being able to lift it means that you are not gaining what you should be from the weight lift.
You should not do the exercises too quickly, as this leads to an uncontrolled movement, which means that, again, you are missing out when it comes to what you gain from your workout. This also increases the chances of injury occurring.
So, the key points to remember are; form is the most important thing in exercise, do not rush the exercise as it may cause you to not complete the exercise correctly, use a smaller weight in order to complete the exercise the right way and feel, and see, the benefits.