I assume that by larger you mean has a diameter which is 3.81 times that of the earth.
Assuming each planet to be the exact sphere, we can calculate their volumes and by comparingvolumes, determine how many earth's can fit into neptune. For example, if the volume of neptune is 10 times that of earth, 10 earth's can fit into neptune.
Volume of a sphere is 4/3 πr^3 = 4/3 π (d/2)^3 = π/6 d^3.
Since neptune has 3.81 times the diameter of earth, it will have a volume which is 3.81^3 = 55.3 times that of earth. Thus, we can say that about 55 earth's can fit inside neptune.
Given the sizes of other planets, we can also calculate the same for them. For example, more earth's can fit inside Jupiter and Saturn.
Hope this helps.