Bond Spread / Liquidity Question security, somebody, why, instead

Iginla2010

Q: Which bond will have largest spread compared to a treasury security?
A) Callable, $20 million oustanding
B) Callable, $80 million outstanding
C) Putable, $20 million outstanding
Answer: A
Ok so obviously choice C can be eliminated right away since it is putable, but why is A the answer instead of B?  The explanation says that smaller issues are less liquid… Can somebody tell me what makes a smaller $ value bond issue less liquid?
Thanks.