Seedlings started indoors tend to be weaker than those that sprout outside in the garden. Indoor seedlings tend to be taller and more tender, even if grown in strong light. The difference is the absence of wind—as wind shakes outdoor seedlings, their stems become stronger and their growth remains stocky.
Indoors, you can re-create the effect of the blowing wind by "brushing" your seedlings. Brush a folded piece of paper across the tops of small seedlings, quickly but gently bending them to horizontal and letting them pop up again. Do this twice a day for about a minute. As seedlings get larger, you can use your hand instead of the paper.
In studies, seedlings that were brushed had smaller, darker leaves that grew closer together than those of seedlings that were not brushed.
Check out Horticulture's Smart Gardening Techniques for tips on seed propagation.