Scout says Miss Maudie's house looks like a pumpkin because the state of the fire burning her house has changed and become more intense. Before this moment, neighborhood men were able to run into the home to save furniture and other items from the flames. For instance, Atticus had the good sense to carry Miss Maudie's cherished rocker out on to the lawn.
When Scout says to Jem that Miss Maudie's looks like a pumpkin because the flames are so orange, this indicates that the fire has entered a new phase. The men can no longer attempt to go inside the house to save goods. The fire has taken over the second floor and is now burning the roof. As Scout remembers it:
window frames were black against a vivid orange center.
At this point, she says,
Jem, it looks like a pumpkin . . . .