Answer: Congratulations on your seedlings!
Seed grown plants often exhibit some degree of variation, even between seedlings started from the same batch of seeds. If your original plant is a hybrid, in many cases the seedlings may even difffer radically from the parent.
Other possible explanations for the larger leaves would include that the seedlings are growing in fresher soil and therefore enjoying the benefits of better nutrients and soil structure, or are growing in less light and are compensating for that by growing larger leaves.
The best way to propagate a coleus plant, if you want to duplicate the original plant, is by rooting cuttings. This method makes "clones" of the parent and the new plants will be identical. It is also relatively quick and easy to do with coleus. Some gardeners will root "slips" or tip cuttings in plain water, others by sticking them in moist soil.
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