POTENTIAL CONFLICT LTSS EVOLVES INTO NMAC ON DEP PROC FROM ARPT WITH MULTIPLE RWY OP PARALLEL RWYS.

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ALL 3 PARALLEL RWYS WERE IN USE (21L; 21C; 21R). I WAS CLRED TO TKOF 21L; FLY RWY HDG (IFR TO GRR) AND TWR ADVISED THEY WOULD CALL MY TURN AFTER DEP. AFTER I ACKNOWLEDGED AND STARTED THE TKOF ROLL; I HEARD TWR CLR AN ACR AIRLINES MLG TO DEPART ON RWY 21C 'FLY HDG 200.' AFTER LIFTOFF; MAYBE 50 FT IN THE AIR; TWR CALLED THE MLG OUT TO ME AS TFC. I ADVISED I HAD THE MLG IN SIGHT AND TWR REITERATED THE MLG WAS FLYING A 200 HDG; TO MAINTAIN A VISUAL AND TURN R TO A 260 DEG HDG. AT THIS TIME THE MLG WAS PULLING INTO A 3 O'CLOCK POS OFF MY R WING. MY AIRSPD IN THE CLB (140 INDICATED) WAS SUCH THAT THE RELATIVE POS OF MY ACFT AND THE MLG WAS NOT CHANGING. WITH A STRONG WESTERLY WIND I NOTED THAT THE COMBINATION OF THE 200 DEG HDG OF THE MLG AND THE WIND WAS CAUSING THE LATERAL SEPARATION BTWN OUR ACFT TO DECREASE. AT THAT POINT I WOULD ESTIMATE 500 FT LATERAL SEPARATION; AND DECREASING. NOT ABLE TO MAKE A R TURN TO 260 DEG; I DECIDED TO INCREASE CLB ANGLE TO SLOW AND FALL BEHIND THE MLG; THEN TURNED TO 260 DEG AND CONTINUED ON AS DIRECTED. IN HINDSIGHT I FEEL THIS INCIDENT COULD HAVE BEEN AVOIDED AND LITTLE INCONVENIENCE CAUSED IF THE CTLR HAD DEPARTED THE MLG FIRST; THUS I COULD HAVE MADE THE APPROPRIATE TURN WITHOUT PROBLEM. THIS SITUATION WAS SUCH THAT I DON'T FEEL THERE WAS ANY REAL SAFETY PROBLEM; HOWEVER; I AM SURPRISED THAT THE CAPT OF THE MLG INITIATED HIS TKOF; OR THAT HE MADE THE TURN TO 200 DEG KNOWING OF THE IMPENDING TFC CONFLICT. I MYSELF WAS SURPRISED THAT THE TKOF CLRNC WAS GIVEN TO THE MLG AS I STARTED DOWN THE RWY!