Lobsters molt, change shells, periodically. When they shed their old shells, the new shell underneath is not fully formed and is still thin and soft.
If you have a lobster with the old shell, the lobster within fills the shell entirely and it's beginning to get too small for him. He sheds it and a new, larger shell is formed. As a result, a soft shell lobster isn't as 'full' as a hard shell. This can be deceiving and you get less meat than you would expect.
On the other hand, a hard shell lobster has a heavier shell so the lobster's weight is a higher percentage shell.
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