The test statistic decreases because your distributions are very similar and larger samples have less noise. If you were to compare the two theoretical distributions you used, you should get the "true" KS statistic. As you add more data, your estimated SK statistic should approach this true value. However, even as your KS statistic decreases, your confidence increases that they are in fact two different distributions (i.e. p-value decreases) because you have greater confidence in your estimates of the individual distributions.

The test statistic decreases because your distributions are very similar and larger samples have less noise. If you were to compare the two theoretical distributions you used, you should get the "true" KS statistic. As you add more data, your estimated KS statistic should approach this true value. However, even as your KS statistic decreases, your confidence increases that they are in fact two different distributions (i.e. p-value decreases) because you have greater confidence in your estimates of the individual distributions.