Each month's total prize fund is a percentage of the interest earned by all eligible Premium Bonds. Therefore, by definition, the newly introduced (April '09) �25 prize reduces the number of other wins e.g. if there are 1 million �25 prizes then there is �25,000,000 less to be won by the other prizes.
The �25 prize was introduced this month. There is now only one �1 jackpot prize instead of two, and your chances of winning are now even less than previously.