Lower Stem keeps Thin

Hi ma Dudes, im a little worried about my Lower stem just being thinner than the rest of the plant. Over all the Girl looks healthy . Also lower stem is Completly Green . Only yellow parts where the little round leaves at the bottom. So what did i do wrong and can do to Fix this Problem ?

That's not what I'd call a problem that needs fixing, if it's healthy ignore it and let it grow on.

If you're really worried you could bury that thin part of the stem after moving into a bigger, deeper pot.

It's not a problem. That happens often. I see that all the time when transplanting seedlings into their final pots and they start growing fast. The top will outgrow the lower stem in diameter for awhile but the lower will catch up.