In the past there was competition within and between disciplines for new start opportunities. The biggest most ambitious program usually won because it was better than the competition, because the number of new missions that congress would allow NASA to start was limited and because large programs tended to require an increase in NASA's budget. The selection of a set of large, infrequent missions had a number of bad effects. It is hard to manage large programs. Big projects are complex and risky.
Cost is non-linearly related to size.
In addition, the infrequent launches associated with large programs meant that space science slowed down. Furthermore, there was great pressure to use the space shuttle both for launch of spacecraft and for flying on board investigations. Now the shuttle is almost completely devoted to assembly of the space station. Nevertheless, because of the long timescales for large missions some large missions remain such as Cassini and the Advanced X-ray Astronomy Facility now renamed Chandra. Moreover there will always be a need for some very large missions such as the Next Generation Space Telescope to replace the Hubble Space Telescope.