My weed plant has a small/skinny stem but gigantic leaves.it falls over cause they are so big?

it falls over cause the leaves are so big.any help?its in a small pot in my window cell.about 1 week old.grew very fast.is about 7 inches tall.i give it enough water so its not to wet in the soil and has nutrient pebbles in the.i gave it rooting formula in the first 2 days.now i am feeding it 6.6 ph water.or so tap water.it has soil mix soil.