Ah okay I get it now, so basically if we decrease r that basically means increasing number of atoms?

No, smaller radius means a smaller sample, that always means less atoms.

For example if we imagine one particle as exactly one atom, then if we decrease its radius, it splits up into many tiny atoms which still occupies the same volume (like the picture above) which leads to the fact that the ratio of the atoms on the surface to the total number of atoms is very very little?

Nope. Atoms are indivisible (that's actually what their name means).

Imagine atoms being small cubes.

Combine them into a larger cube 10x10x10 - how many atoms there?

How many of them are on the surface? How many of them are inside?

What is the ratio of these numbers?

Now combine identical atoms into a smaller cube 5x5x5 - how many atoms there?

How many of them are on the surface? How many of them are inside?

What is the ratio of these numbers?

For which cube - the large 10x10x10, or the small 5x5x5 - is the ratio of those on the surface to those inside larger?