The device is a metaphor. Bob Ewell is being compared to a small but fierce chicken which was often used in cock fights. The suggestion is that Ewell is an angry person, ready to fight as a moments' notice but he was really a small man with little power. His "strutting" like a chicken, would indicate a sort of display of bravery designed to scare others off. However, since is a "little bantam" ( or lightweight) man, his strutting is not taken seriously.
The literary device Harper Lee is using in this situation is metaphor, the comparison of two things that have similar characteristics but which are not the same. The author does not use "like" or "as", which distinguishes her use of literary device from simile. The author is comparing Bob Ewell to a bantam rooster, because of his small physical build but extremely feisty, combative demeanor. The metaphor also presents strong imagery, another literary device. Through her choice of words, Lee enables the reader to visual Bob Ewell, a small man itching for a fight with everyone around him, lacking in stature but overflowing with belligerent confidence like "a little bantam cock", his chest thrust forward and head held high, stepping confidently up to the witness stand (Chapter 17).
Scout's description of Bob Ewell as a bantam rooster is a metaphor, a comparison between two unlike people or things that equates the two.
Bantams distinguish themselves as displaying a bold nature beyond what should be expected from such a small chicken. Sometimes, though, it is merely a false bravado. For instance, one breeder describes many of the roosters as having "the Napoleon complex" and adds that he does not allow these aggressive roosters to remain on his farm. However, he once had two that he felt he would have to send off until he observed something unusual about them:
The funniest situation was when two same-aged roosters would play the respect game, only to go to roost together at the same time in the evening, even while the hens pecked around the outside pen.
This description of the two bantam roosters who merely posture is truly an apt description of Ewell, as well. He, too, merely postures as he does when he follows Helen Robinson to the home of Link Deas. Moreover, Ewell's mere pretense of aggressiveness is the reason that Atticus does not fear that Bob Ewell will harm him or his family after this "bantam rooster" spits tobacco in Atticus's face. Another reason Atticus does not fear Ewell is the fact that he has never been convicted of any assaults. In chapter 17 Judge Taylor says that he does not recall ever seeing Ewell in his court.
As it turns out, however, Atticus misjudges the reprehensible Ewell who, while he is not a bold "bantam rooster," nevertheless commits an aggressive act as he sneaks up on the innocent Finch children and tries to serve his revenge on Atticus by harming Jem and Scout.