How do I up-scale a transistor
Short Answer: Drive it harder with up to 10% load current.

The important thing to remember about BJT switches is Vce(sat) is rated at Ic/Ib=10 and is much less than hFE , (often 10% hFE for good saturation)

Your BD136 will work fine with Vce=0.28V @ 1A and Vce=0.225 @ 710 mA but only if Ib=10% of Ic.  Thus using the figure for Vbe(sat)=0.9V vs Ic @710mA and the actual Vol of your 555 at 70mA compute the lower Rs needed to get ~70mA base current. 470 ohm is too big. 

This may or may not work with a CMOS 555 rated for 50mA out.

But at 70mA out Vout low=2.5V + 0.9V Vbe leaves 12-3.4=8.6V then /70mA = 123 Ohms @ 602mW so try 180 ohms 1/2W then 150 or 123 with 1W if it is not bright enough.

The BD136 from above is equivalent to (0.28-0.225V)/(1A-0.7)= 0.183 ohm.  An alternative is to choose a FET with RdsOn&lt;&lt;0.1 ohm.  The smaller, the cooler.