I think Sophia was actually bustier.
Was, and is, actually. But I do wonder why Ms. Loren was so angry at Ms. Mansfield that evening 
Finally, we have the answer!
Vanity Fair's Hollywood: "
Sophia Loren Finally Tells the Story Behind That Infamous
Jayne Mansfield Photo"

"It’s hard to pinpoint when, exactly, the first documented instance of side-eye appeared. But many credit this 1957 photo of
Sophia Loren and
Jayne Mansfield. Newcomer
Loren had just skyrocketed to stardom in Europe when she became
the most photographed performer at the 1955 Cannes Film Festival. The Beverly Hills party two years later was meant to officially welcome
Loren to Hollywood. But the Italian bombshell found her spotlight unexpectedly stolen by
Mansfield. In a recent interview with Entertainment Weekly,
Loren confirms that, yes, she was thinking exactly what it looks like she was thinking.
"Paramount had organized a party for me. All of cinema was there, it was incredible. And then comes in Jayne Mansfield, the last one to come. For me, that was when it got amazing. . . . She came right for my table. She knew everyone was watching. She sat down. And now, she was barely . . . Listen. Look at the picture. Where are my eyes? I’m staring at her nipples because I am afraid they are about to come onto my plate. In my face you can see the fear. I’m so frightened that everything in her dress is going to blow—BOOM!—and spill all over the table."Loren went on to explain that even though she was caught smiling or having a good time with
Mansfield later in the evening, her mind was always on that dress.
"No, no. Well, there may be other photos, but this is the picture. This is the one that shows how it was. This is the only picture."But the veteran actress, who has more than 90 film credits to her name, says that even if she
was thinking all those things, she doesn’t like being associated with the photo that
launched a million side-eyes."Actually, many, many times I am given this photo to autograph it. And I never do. I don’t want to have anything to do with that. And also out of respect for Jayne Mansfield because she’s not with us anymore."In 2012, two-time Academy Award winner
Loren told Vanity Fair she was reluctant to give interviews. "My life is not a fairy tale," she said, "and it's painful still to speak about it." But
Loren seems to have had a change of heart. She has written a memoir titled
Yesterday, Today and Tomorrow: My Life, which was published this month. After irresistible phrases like,
"Everything in her dress is going to blow," who
doesn't want to read more about
Loren's reign during the golden age of Hollywood?