How can Group Size offer more Success, but not? (EX)

Yay: Larger Membership can be more Important in Persuading Government as then the Pressure Group show off a Significant Size of Population. Example: Campaign to Protect Rural England had 40K members and has Defended the Green Belt from Property Developers Nay: Large Groups doesn’t bring Success always. Example: Campaign for Nuclear Disarmament had 110K members in mid 1980s but had little Influence on Government