Seedlings often grow up long and thin when they get too little light and then stretch towards the light. Too high temperatures, especially due to heating inside the house, also stimulate strong growth. Furthermore, seeds sown too densely can also lead to excessive growth.
So this is what you can do: Place your plant pots or seed trays close to a window, for example on the windowsill. Make sure that the seeds are not sown too close together to give the plants enough space to develop. You can carefully prick out plants that are growing too densely. As soon as the last frost is over, the plants be transplanted outside. If it is still too cold at night, but quite mild during the day, you can place the seedlings in the fresh air during the day to accustom them to the cooler temperatures.