Lets say a person is born with both thumbs unable to "bend" more than 5 degrees at the MCP joint (center or middle joint).
This picture is of a thumb bending well over 45 degrees at the same joint. 
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1 Answer
I think you are referring to thumb hypoplasia. Or more specifically thumb hypoplasia Type II. It restricts the movement of the thumb. It is a congenital disorder. The cause is still unknown, maybe some genetic deformalities.
Type II is characterized by a tight web space between the thumb and index finger which restricts movement, poor thenar muscles and an unstable middle joint of the thumb metacarpophalangeal joint. This unstable thumb is best treated with reconstruction of the mentioned structures. [From: Wikipedia]
Further reading:
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Thumb hypoplasia refers to an abnormally small, poorly developed thumb. Type 2: Small bones and muscles with an unstable thumb joint. OP hasn't mentioned the hand being small...– NarusanCommented Jun 2, 2018 at 13:15
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@Narusan Nor has they mentioned the hands being big except the photo :) But as they said "born" ,I presumed it as being small. Commented Jun 2, 2018 at 13:23
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1Well, it could also have to do with a naturally short flexor tendon, if the left picture is almost the only possible hand posture, it looks like a congenital clasped thumb. I think without X-rays etc. the question is too broad.– NarusanCommented Jun 2, 2018 at 13:31
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1+1: Ah, now I see, you got the idea from "underdeveloped". That might actually be true. It's not a very well formulated question, tbh.– NarusanCommented Jun 2, 2018 at 13:47