Air temperature changes will cause the air and fuel vapor level to change in the tank. The vent system is designed to maintain a slight positive pressure (~1PSI) in the tank for emissions purposes. When the fuel system runs, the fuel is also heated both by the pump and by the engine. The amount returned to the fuel tank helps to heat up the rest of the fuel which then expands and causes more vapor. Excessive pressure is vented directly to atmosphere in the older diesels, or through the charcoal canister on gasoline vehicles.

If you want a great visual, take a plastic jerry can and fill it half full of gasoline then seal it tightly. Keep it in a cool shady place overnight, then go place it in the sun for ~30 mins an come back. It'll be puffed up like a balloon.

You can also have a very slight vacuum on the tank depending on conditions and fuel consumption. When you pull the cap off, you hear that whoosh - it can be positive pressure OR vacuum. Keep in mind that 1PSI can feel like a LOT of pressure when it really isn't.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)