If the bottom part of a dress is too tight but the top fits perfectly, there are a few options to consider:

Alterations: You can take the dress to a tailor or seamstress to see if they can let out the seams on the bottom portion of the dress. This will give you some extra room and make the dress more comfortable to wear.

Layering: You could try layering a skirt or petticoat underneath the dress to create more volume in the bottom portion. This can help to alleviate the tightness and make the dress look more flattering.