At that age variations in weight are not a big deal unless there is visible issue in most cases. If you are worried check it out with a doctor in case there is an underlying medical issue.
If the child is eating well, healthy and active a few kilos is not a big deal. I was tiny all the way through because my genetics make me so, my son is the opposite, so we have been very different percentiles and are different build but our weight is not a problem.
