head lag ?

This is my baby. She’s 5 months and I noticed she had a slightly larger head than most babies and than her body. She holds it up she still drops it down sometimes like her head just gives up on her if that makes sense ? As if she’s tired of holding her head up. She holds her head up if we hold stuff up to her and she’ll look up but she doesn’t seem to hold it up very good for a long period of time. Is it normal ?