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.