Scallops have been called the “marshmallows of the sea” because of the physical resemblance. “Some gourmets claim that the tiny bay scallops, which resemble miniature marshmallows are more tender, delicate and sweeter. Others favor the flavor and texture of sea scallops, which are similar in size to regular marshmallows” was printed in the Tampa (FL) Tribune on August 27, 1987.
   
“Nantucket bay scallops are drizzled with olive oil and sea salt; they’re the marshmallows of the sea” was printed in The Record (Hackensack, NJ) on February 6, 2009. “Scallops really ARE the marshmallows of the sea!” was posted on Twitter by jim sharp on May 10, 2009. “Scallops are the fluffy marshmallows of the sea” was posted on Reddit—Showerthoughts on March 8, 2019.
     
“Scallops are just savory marshmallows” and “Scallops are the tater tots of the sea” is a similar saying.
   
       
Wikipedia: Scallop
Scallop (/ˈskɒləp, ˈskæləp/) is a common name that is primarily applied to any one of numerous species of saltwater clams or marine bivalve mollusks in the taxonomic family Pectinidae, the scallops. However, the common name “scallop” is also sometimes applied to species in other closely related families within the superfamily Pectinoidea, which also includes the thorny oysters.
 
Scallops are a cosmopolitan family of bivalves which are found in all of the world’s oceans, although never in fresh water.
       
27 August 1987, Tampa (FL) Tribune, “Sweet scallops chase away dinnertime blahs” by Gay Starrak, pg. 11-G, col. 2:
Some gourmets claim that the tiny bay scallops, which resemble miniature marshmallows are more tender, delicate and sweeter. Others favor the flavor and texture of sea scallops, which are similar in size to regular marshmallows.
   
6 February 2009, The Record (Hackensack, NJ), “Eating Out in N.Y.C>: The John Dory” by Ryan Sutton (Bloomberg News), Better Living, sec. F, pg. 27, col. 2:
Nantucket bay scallops are drizzled with olive oil and sea salt; they’re the marshmallows of the sea.
   
Twitter
drunkbrunch
@drunkbrunch
Rebecca: “I call scallops the ‘marshmallows of the sea.’”
2:26 PM - 1 Mar 2009
   
Twitter
jim sharp
@DeoLaudamus
Scallops really ARE the marshmallows of the sea!
3:14 PM - 10 May 2009
   
Twitter
🦄 🍭Kim
@passthepocky
Little bay scallops are like mini marshmallows of the sea.  Deep huh?  😊
6:04 PM - 20 Jun 2009
   
Twitter
Liz
@pensivefibers
“Scallops are sometimes called the marshmallows of the sea.” - Good Eats
2:06 AM - 21 Jul 2009
 
13 April 2013, The Gazette (Montreal, Quebec), “Casual Restos” by Sarah Musgrave, pg. J5, col. 2:
For mains, we were treated to the first scallops of the season from Iles de la Madeleine. Amazingly sweet, roasted for a delicate crusting on the ends, they proved again to be marshmallows of the sea.
     
Twitter
[ 🦈17’S KELSEY ⭐]
@kelsey1344
I have just learned that sometimes scallops are called the marshmallows of the sea. Thank you, Alton Brown.
12:06 AM - 19 Oct 2013
 
Google Books
Talking Turkey:
A Food Lover’s Guide to the Origins of Culinary Words and Phrases
By Robert Hendrickson
New York, NY: Skyhorse Publishing
2014
Pg. ?: 
the marshmallows of the sea. A name sometimes given to ocean scallops because of their shape.
   
Google Books
The Laws of Cooking:
And How to Break Them
By Justin Warner
New York, NY: Flatiron Books
2015
Pg. 71:
Scallops are pretty much the marshmallows of the sea.
 
97.3 The Dawg (Lafayette, LA)
SCALLOPS, THE MARSHMALLOWS OF THE SEA – FOODIE FRIDAY
BRUCE MIKELLS November 27, 2015
People always looks at me funny when I call scallops “the marshmallows of the sea”. I just have always called them that. They look like marshmallows and they are some of the sweetest treats that mother ocean offers us who walk the land.
     
Austin (TX) Chronicle
The Dish: Relish the Comfort of Hibachi Scallops at Sushi Japon
Newly remodeled Japanese steakhouse delivers entertainment and big flavor
BY JESSI CAPE, FRI., OCT. 19, 2018
(...)
I recommend ordering the scallops because there’s really nothing more decadent than those seared little marshmallows of the sea.
   
Twitter
Justin Hennen
@Just_Hen
Scallops are the marshmallows of the sea
2:48 PM - 15 Dec 2018
 
Reddit—Showerthoughts
Posted byu/Columbusquill1977 March 8, 2019
Scallops are the fluffy marshmallows of the sea.