A larger size isn't going to help you and the cost of fitting this to you will cost more.
Adding a ruffle is an alternative but I wouldn't do it as it changes the overall appearance.
Return it, sacrilege! You bought this dress because you like how it looks and I'm sure it looks wonderful on you. Hands down it's a fun dress! So really what can you do?
As a seasoned seamstress I recommend
Removing the solid blue waistband and increase the band width similar to the Tiki Room. I don't own the Small World dress but I own several others. I assume the blue waistband measures between 1.5-2" wide. The Tiki room waistband measures a whopping 4". The best part of replacing the waistband is you can increase it to your needs without compromising the look of the dress.
Waistband Comparison
Small World / Tiki
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With this plan, here's what it should look like...
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