My cavalier just turned one last month and is still the same way. We stood him next to the standard size and he is much much taller with gangly legs and a skinny little body. Something to keep in mind though was that his dad was the largest they come in the breed and apparently there are three sizes of cavaliers.
Yes! Cavalier go through a 'skinny phase' as they are growing due to their increased size/height without fully filling out. Your dog will fill out as it matures, but being skinny at that age is very normal for Cavaliers since they have such long bodies/legs.