Since it's difficult to determine why a company is buying back loads of it's own shares, lets look at some recent buybacks in terms of scale:
Johnson & Johnson (today), market cap = $183 B
SHARE BUYBACK = $10 B
Home Depot (June 19), market cap = $80 B
SHARE BUYBACK = up to $22.5 B
Wal-Mart (June 1), market cap = $200 B
SHARE BUYBACK = up to $15 B
ConocoPhillips (today), market cap = $137 B
SHARE BUYBACK = $15 B
Recent findings which appeared in the Journal of Financial Economics indicated that companies who buy back their own shares outperform the market by an annualized average of 3.1% over the next four years, after the repurchase.
I welcome any other recent examples, especially any that eclipse Home Depot's 28% of market cap.