The phrase "small opening" refers to a small space or entrance that allows access into something. It generally implies that the space or entrance is limited in size or narrow. Full definition
Keep in mind that cats can jump over gates and squeeze through small openings, so you'll want to monitor him. (caninejournal.com)
Smaller bonuses may be available with smaller opening deposits. (fool.com)
This is a 30 - inch tall gate covers openings between 29 and 44 inches wide and also has a 10 by 7 inch smaller opening on the bottom of the gate. (whatbabyneedslist.com)