Keep in mind that until New Horizon gets there we don't know a great deal about the atmosphere of Pluto. There seem to be a number of theories (the article I'm using as a source doesn't match the paper linked in a comment) but our only way to observe the atmosphere so far has been during infrequent occultation events when Pluto moves between an observatory on Earth and a relatively bright star allowing the star to be viewed through the atmosphere. That can tell us a good bit, but the first observed occultation was in 1988 and Pluto has an orbital period of about 248 Earth years, so we've only been viewing the atmosphere near perihelion (which happened in 1989). That in turn means we've never actually viewed the atmosphere at apohelion so we don't actually know what it does there. We can model it, but we know little enough pre-New Horizon that I'd expect the models to change as more data comes in.
That said, I'm using this article: Lifting the Veil on Pluto's Atmosphere, which presents a broad overview of what we know from observation, what we suspect from modeling, and what we hope to discover from New Horizon.
Pluto has an eccentric orbit: it goes from a perihelion of 30 AU to an aphelion of 50 AU with temperature swings from -220 degrees Celsius to -240 degrees Celsius. The low temperature is cold enough for the gasses that make up the atmosphere to freeze and "snow" to the ground in solid form. It's unclear this actually happens because we haven't observed Pluto at aphelion. From the article I linked above:
Scientists originally thought that as Pluto recedes from the sun, and the temperature decreases to about minus 240 C (minus 400 F), the vapors freeze and fall back down to the dwarf planet's surface. However, observations made as recently as 2013 and coordinated by the Portable High-Speed OccultationTelescope group from multiple sites including the 0.9 m astrograph at Cerro Tololo Inter-American Observatory (CTIO) and the 1 m Liverpool Telescope on the Canary Islands, indicate that Pluto's atmosphere is not collapsing, but rather thickening.
So, although Pluto's atmosphere gets thicker and thinner through its orbit of the sun, it may never completely freeze out and "collapse."
Michael Summers, New Horizons co-investigator and member of the atmospheres science theme team, said it's too early to tell whether Pluto's atmosphere freezes out or persists through its orbit. This makes sense, as Pluto has made only one-tenth of an orbit around the sun since the discovery of its atmosphere in 1988.
To answer your question directly: As Pluto warms at perihelion the atmosphere grows because the gasses are warming and expanding (or melting from solid form). At aphelion the opposite occurs as the gasses drop in temperature and either freeze or just become more dense.
For fun (and to demonstrate a gas reducing volume due to cold), here's a video of dipping a balloon in liquid nitrogen:
