My hormone levels (testosterone/estrogen) are normal. i dont use steroids or medications. doctor said i have gynecomastia (glandular tissue growth). i am a fit male, but my cardio is bad ie. running. what could be the cause for my breast development?

Happens often in young men for reasons that are not known. I would discuss with Drs over there as may be causes that we don't see here. To learn details and it can be complex I would go to uptodate.com and spend some guilders for a month's subscription (Euros? I can't recall) and learn all about it. I don't know what it means "cardio is bad i.e. running" over there. In general running is good.