For low numbers we rely on our physical understanding. I can see three cups and I can throw 5 cricket-balls, For high numbers this is not possible and other capabilities take over. Consider:
The second number is much larger than the first. But I have no physical appreciation of this fact. (It is possible to increase this by training in quantitative physics); but here its much simpler as we can rely on the same low number paradigm as before. I don't actually see the number physically, but I do see quite quickly that the second has more digits than the first. Now lets consider the following:
The second is much larger than the first, now reading the number of digits is something that can't be done at a glance, but looking at it geometrically, one can judge that the second is perhaps ten times as long.
Practically, one has to consider that large numbers are fairly pointless. There are after all only about a googles worth of particles in the universe. One can write this number down - 1 with a hundred zeroes after it. Most of the time we actually deal with quite small numbers - 6, 30 maybe 500,000. Hence, for most people, for most of the time, a fairly primitive understanding of quantity is all thats required: nothing, one, two, a little and a lot.
The picture becoes clearer when we begin to look at cardinal numbers. Here there is no hope of usingphysical imagination, and what one uses is how they function within the body of mathematics, that is by the statements that use it,and are understood by it. In Saussearean terminology we understand it structurally. That is by an element in a whole, and its relations. One could develop from here , I suppose, a structural epistemology of quantity.