What does it mean if your baby is measuring small
It all depends on the age of your baby and if you mean outside the womb or inside.
Outside the womb....
Could mean your baby is just small. I had three smaller than average babies. With the first the pediatricians had me go through tons of hoops (including feed my 5-6 month old vegetable oil!) to see if she'd gain weight. It was ridiculous. I never did that again and just nursed and nursed my other two with my husband calmly telling me he was smaller than average as a baby and child so they probably just took after him. Introducing solids when they were ready.
It could mean you need to feed baby more formula or have baby at the breast more to increase your milk supply.
It could mean something else...your doctor will be able to advise you, but also follow and trust your instincts, if they differ from your doctor. Our instincts with our first small baby were screaming "no" at the things the doctors were suggesting but we did them anyways because we were concerned for our daughter, to appease the doctors and "prove" we were good parents. I wouldn't do it again because I am a good parent when I advocate for my child's best interest too.
Assuming the babe IS growing and happy and eating.. let them be.. Gosh we worry so much..