The raisin bread model of the expansion or the universe helps in getting an intuition of what is happening:
The specific form of Hubble's expansion law is important: the speed of recession is proportional to distance. Hubble expressed this idea in an equation - distance/time per megaparsec. A megaparsec is a really big distance (3.26 million light-years). The expanding raisin bread model at left illustrates why this proportion law is important. If every portion of the bread expands by the same amount in a given interval of time, then the raisins would recede from each other with exactly a Hubble type expansion law. In a given time interval, a nearby raisin would move relatively little, but a distant raisin would move relatively farther - and the same behavior would be seen from any raisin in the loaf. In other words, the Hubble law is just what one would expect for a homogeneous expanding universe, as predicted by the Big Bang theory. Moreover no raisin, or galaxy, occupies a special place in this universe - unless you get too close to the edge of the loaf where the analogy breaks down.
The expansion of the dough in the oven , in which the raisins are embedded does not affect the size of the raisins. That is because the dough is very elastic and expands, while the raisins are held be much stronger cohesive forces.
For the same reason , the effective "force" of expansion due to the Big Bang is very much smaller than the gravitational attraction that holds galaxies together so does not affect their size. It is more so for atoms and matter as we know it which is held together by forces orders of magnitude larger than the gravitational force.