Answer: Bell peppers grow best in the heat of summer, with as much sunshine as possible. Some varieties are naturally small; some are naturally large. I don't know which variety you are growing or how much sunshine they're getting so I can't really offer any more information. Next year, plant your peppers as early in the season as possible and site them in the sunniest spot available, preferably next to a foundation, wall or fence where reflected heat will help them develop the earliest, largest peppers possible.
Best wishes with your garden!
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