Size 7 diapers are meant for those children that are above 35 lbs in weight.
My nephew who is 7 years old fits easily into size 7 diapers and wears them for bed wetting or on long road trips where he will fall asleep and wet his pants.
The 7 year old nephew still has some room to grow in the size 7 diapers and they will probably still fit him until he's at least 9 or 10 years old unless he gets bigger in the waist etc.
Some 3 year olds are in size 6 and size 7 diapers while some 3 year olds may still fit in size 5 diapers.
It all depends on the child and the way they are built.
Some children are taller and bigger in the waist than others so the diapers can fit different children at different sizes.