Does it matter which union is the 'largest'?

Inter union rivalry has been a negative characteristic of British trade unionism for more than a century. The highly unionised teaching profession is strongly contested by 3 TUC affiliated unions and claims of the 'best', 'biggest' and 'largest' trade union feature on union websites and other communications. The NUT recently lodged a complaint with the Advertising Standards Authority (ASA) about an NASUWT job advertisement which included a claim to be 'the largest teachers' union in the UK'. The complaint was upheld and the NASUWT must not repeat the advert in its current form. The ASA has told the NASUWT to ensure it holds evidence to support such claims in future.
    A parallel situation arguably exists between Unite and UNISON with the former claiming to be 'Britain's biggest trade union' despite having 150,000 fewer paying members. But does size matter? The Con Dem Government seems to think so with Part 3 of the Transparency of Lobbying, Non-Party Campaigning and Trade Union Administration Act 2014 bringing in a new statutory requirement for unions to submit a membership audit certificate (verified by an independent assurer) to the Certification Office each year. The pretext for this legislation was that unions 'should reflect the will of their members' but in reality it is about marginalising unions from public life. And faced with such hostile state interference maybe it's time for unions to end one-upmanship and place greater emphasis on trade union unity rather than making competitive claims more befitting of mobile phone firms and supermarket chains?
http://asa.org.uk/Rulings/Adjudications/2015/1/NASUWT/SHP_ADJ_261873.aspx#.VNUtNlZFDIX