What Causes Enlargement Of Left Side Of The Skull And Lumps On Either Side?

Since a few months old, the left base of my daughters skull has been twice the size of the right. When I feel the bottom of my skull, I feel two "round" "lumps" on either side of the spinal entry at the base of the skull. They are symetrical and evenly sized. My daughters are symetrical but the left rounded area is bigger. Is this cause for concern? She is now 18 months and this is still the case so I don't believe it is a swollen lymph node.