Standard sizes for mattresses vary up to 1 inch. Innerspring units inside the mattress are the same size, but depending on the bedding model and materials used, the finished size could differ. 

The filling material is layered and rolled over the spring border wire. Since a lower-end mattress has less filling material, the narrower and shorter the mattress will be. In contrast, a higher-end mattress has more filling material, therefore the mattress will be slightly wider and deeper.