Halo sleepsack

So my son has started rolling back to belly along with being able to break his way out of a swaddle which means no sleep for mama 😫 I ordered a halo sleep sack size medium because he’s close to 20 pounds and over 24 inches (not 100% on length our 4 month apt is Monday).. but it is HUGE! He’s literally swimming in it, like it extends a foot at least below him; my question is, is that how it’s meant to fit? Or should I return and order a size small? Also to those who use the halo sleep sack do you feel it helps sleep?