Why do classes with fewer words have higher probability?
The Naive Bayes formula doesn't do what you say. For classes $C_k$ and words $x_1, \ldots, x_n$, the formula is

$P(C_k | x_1, \ldots, x_n) \propto P(C_k) \prod_{i=1}^N P(x_i | C_k)$

If the word newword doesn't occur, we have no prior information about $P(x_i | C_k)$, so all these conditional probabilities should be equal. So the right-hand side is just proportional to $P(C_k)$, which is independent of the number of words in $C_k$.