What you said is exactly wrong. A good couplet should have the upper couplet lighter and the lower couplet heavier.
We know that a couplet consists of upper and lower couplets. If the first couplet is written with strong momentum (heavy) and the second couplet is written with weak momentum (light), it will give people a feeling of anticlimactic, which is called "heavy at the top and light at the bottom." Heavy at the top and light at the bottom is also one of the symptoms of couplets.