bark is worse than his bite
A person seems more hostile or aggressive than is the case, as in Dad sounds very grouchy in the morning, but his bark's worse than his bite. This phrase was a proverb by the mid-1600s.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.