Many observers had expected SMC, which has spent much of the past 12 months gobbling up architecture practices across the country, to jump straight into the AJ100 top spot, but statistics show that it only employs 270 architects compared to BDP's 301.
This means that BDP, which has held on to the top berth in the AJ100 for the past seven years, is 10 per cent bigger than its closest rival.
Nonetheless, the rise of SMC, which was formed by Stewart McColl in 1996, has been incredible, particularly considering the firm was unranked in last year's AJ100.
That growth has been fuelled by an aggressive acquisition policy that has seen it snap up 14 practices in just over five years - among them Corstorphine & Wright Kenzie Lovell, acquired in 2002, and Alsop Architects, which was bought last year.
SMC is one of only two architecture firms listed on the UK stock market, the other being Aukett Fitzroy Robinson, which is ranked 15th in this year's AJ100.
However, SMC's impressive streak of acquisitions was brought to a sharp halt by an unexpected boardroom shake-up in February, which saw McColl moved into the deputy chairman's role to make way for former non-executive chairman Rodney Walker, who now controls the day-to-day running of the business.
The reshuffle came after the firm posted a profit warning that downgraded expected profits from £7 million to £5 million. Walker put further acquisitions - including the expected purchase of S&P - on hold. 'In the short term we will be focusing on realising operating efficiencies and organic growth,' he said.
If SMC had bought S&P, which is ranked 85th in this year's AJ100, it would have added 32 architects to its books, giving it a total of 302, compared to BDP's 301.
And BDP chief executive Peter Drummond said he is not complacent about the year ahead and admitted that the firm would have to be 'resilient' to maintain its steady growth, and keep SMC at bay.