The seedling is not even 3 inches tall and has a leaf bigger than itself. This is a polyembryonic honey mango seed. It gets 1 hour of sunlight daily and the pot has no drainage holes. I also am probably overwatering it. I water it once a day.
I wouldn't be concerned about the size of that one leaf, it looks healthy, but it is a concern that you have it in a pot without drainage holes. Since you mention it yourself, you probably already know it shouldn't be in something that doesn't have drainage holes, so I'd recommend you move it into a pot with holes; the difficulty is deciding when to do that so as not to cause damage to fragile new roots. You may need to wait until it grows on a bit more, unless it starts to show problems related with poor drainage.
Information here about growing mango from seed - you will need to scroll down a fair way to get past the 'how to germinate' section https://empressofdirt.net/grow-mango-seed/