"Let not him that eateth [the strong] despise him that eateth not [the weak]; and let not him which eateth not [the weak] judge him that eateth [the strong]...."
Paul points here to two different sinful tendencies for the strong and the weak:
First, the sinful tendency of the strong toward to weak is one of despising (exoutheneo: to treat with contempt or to look down upon). The strong tend to say or think about their weaker brethren: "Can you believe how uptight, narrow, or legalistic that weak brother is?"
Second, the sinful tendency of the weak toward the strong is one of judging (krino). The weak tend to say or think about their stronger brethren: "Can you believe how liberal, slack, lacking conviction that strong brother is?"
Murray notes: “Both are condemned with equal vigor.” He continues, “In actual practice these vices appear respectively in the smile of disdainful contempt and in the frown of condemnatory judgment" (Romans, Vol. 2, p. 175).