This has been bothering me so I went on the Speedo web site and cannot find anything about a "Speedo Life Suit" but there was something called a "Bubble Float Suit" and is available in childrens sizes 2-3 and up to 5-6.
Its almost like a scub one piece suit and has floatation devices in it to keep the child afloat. However it does go by size.

Also my niece was very tiny, thin for her age and had a hard time finding flotation deviceed to fit her when they took her out on their boat. She was so lightweight that when they put a vest on her, she would fall over because of the weight...maybe this is what happened with your child...not sure but I do not think its Speedo's fault really. I just think it was a suit that did not fit her right.