That's one of the reasons they put vent holes in tanks. It's not just to allow air to excape while filling the tank. It's also to allow for expansion and contraction from temperature plus it also allows fuel to flow to the engine without creating a vacuum in the tank.
As to the more gas in the tank then when first filled comment.. no, there isn't. Assuming the vent was closed, (and we know it was since you said the gas was under pressure) if you weighed the tank when it was first filled up and then after it sat in the sun, you would find the same weight..but what you would have is gas that took up more volume to achieve that same weight. The molecules in the gas got further and further apart as they absorbed heat, causing the gas to expand and take up more room.
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God) 
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.
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