This should probably be "I've ____ed since ____":

I've dropped 20 pounds since the beginning of the year.

Correct grammar dictates that we should "have ___ed" whenever we specify the time period using "since" or "for":

I've been practicing for over 3 years.

I've been waiting since nine o'clock.

But sometimes English speakers make mistakes when speaking and use the simple past tense instead.