As adults, we can quickly pick out the longest, tallest, shortest, heaviest and smallest items but as with so many abstract concepts, these have to be learned by our little ones. Learning about comparative size is made all the more complicated because objects seem to change. For example, take an egg and a chicken - the egg is small, the chicken is large. Now add an osterich, suddenly the chick is small and the osterich is big. How did that happen? The chicken didn't suddenly shrink!
We are exposed to different sized objects every day. Talking about different sized objects will instil these abstract concepts. Many picture books also explore concepts of size. Look out in your stories and in the real world for examples and talk about them with your toddlers. Play games - take different toys and sort them by size, sort pieces of string or balls. Talk about the size order of food items on your plate - sliced carrots are bigger than peas, but smaller than boiled potaties. All of this will help them to cement their understanding.