By scaling it up, we mean that we increase each of the three linear dimensions by the same factor, making a larger copy of the small animal. If the scale factor is two, then a leg will become twice as long and twice as wide. The eye will increase in size to twice the diameter of the original eye. So, the spherical body of the larger animal has twice the radius of the smaller original. If their is not some compensating factor involved, this would have to have serious consequences for the physiology of the animal.
Many physiological phenomena depend up the amount of surface area or the amount of total volume.
For example, imagine that our animal has no gut and absorbs nutrients across the external body wall. If we scale up the animal to twice the radius, we will have four times as much surface area over which to absorb nutrients. But, we will have eight times as much volume of cells which are using these nutrients. So, as the animal becomes larger in scale, each cubic millimeter of volume will have to share less and less of the nutrient material obtained from diffusion in through the body wall. The conclusion we come to is that the body plan must change as the size of an organism increases.