Thats BrianG's 'no-spark' connector:
http://www.scriptasylum.com/rc_speed/_nospark.html
As for the size of the spark, that depends on if the caps are discharged or charged when yuo plug in the batts, and the size of the batts (voltage wise). Sometimes I get sparks, sometimes I dont- I wouldnt worry about it unless its being caused by reversed polarity between the batts and esc...
