Up until a few days ago I believed the same thing. But the problem is, I can't lift heavy. I'm seriously really weak. It was starting to get to me that I couldn't squat my own body weight, or bench press 200 pounds.
But then I realized something. I lift MY heavy. Do I lift as heavy as I can? Yes. Do I push myself to lift 5 or 10 pounds heavier than what I normally do? Yes. But I don't let the actually numbers on the weights bother me anymore. Obviously my body is toning and defining and getting better by lifting what I consider heavy.
And also, I'm training to compete in the bikini division, not the figure or bodybuilding division. According to the NANBF the standard for bikini division is-
"A specific level of muscularity or leanness is not sought after for NANBF Bikini competition. Judging is based on having an overall healthy-looking toned body with good symmetry, shape, fullness, balance and proportion with good poise, posture, confidence, stage presence and comfortable presentation. Other factors that can affect overall appearance are skin tone, complexion, suit fit, hair and make-up. Bikini competitors should not display an abundance of muscle size, deep muscle separation, or muscle striations."
Source -- NANBF
Which means that heavy lifting isn't definitely necessary. Don't worry if you can't lift heavy. Just lift. Challenge yourself. If a challenge means going from 15 to 20 pounds that's fine! On a normal day I only squat 30 lbs and all my arm and shoulder work is done with 10-20 pound dumbbells. But, my muscles are responding to it. Which is all that matters. One day I'm hoping to be able to lift heavy but, until then, I'm perfectly happy just lifting.