Physical milestones refers to your child's height and weight. Typically at this age, boys will be between 37 and 45 pounds and 41 to 44 inches tall. Girls will be between 36 to 44 pounds and 41 to 44 inches tall. While these are just ranges, and your child may fall under or above these ranges and there may not be cause for concern, it is important to discuss this with your child's pediatrician so that your child's pediatrician can track your child's development and make sure it's on track.
While you may notice that in the playground or at school your child may be shorter or taller than others, and you may be concerned as to why that is, and you may think to yourself, "Well I don't recall being taller or shorter than anybody else. I feel that I was average. I don't know why my child is." It's OK. Kids develop at a different rate, and sometimes girls even develop faster than boys typically, and they might even catch up to boys.
So what's really important is that you're monitoring how your child is growing in terms of weight and height. That means speak with your pediatrician and make sure that you have a copy of their growth chart so that you too can continue to monitor your child's growth and height.