My legs are slightly different lengths. I didn't know, but the woman who did my bike fit told me so. Also, if your top tube is too long, you may be stretching your arms and locking your elbows. Even if your top tube is fine, you may still be locking your elbows.

Posture is just as important on the bike as off. Also, strengthening the core muscles will allow you to hold more weight with your core, and less on your arms and hands and shoulders.