it don't change the fundamental meaning.

In the example you provide above, it's not いんです, but rather the い-type adjective 軽い + んです. They both end in んです＝のです (の is contracted to ん), which can follow verbs and い-type adjectives as is, but need to add な if following a noun (誕生日) or a な-adjective (like きれい or 静か). The meaning of sentence final のだ/のです differs depending on the context, but one common use is to imply that what you are saying is an explanation for a previous context.

from what i have learned, it does not change the fundamental meaning, the format ん