Sentence:  While the little MTU Vee-12s in the Class 800 bi-mode generating units have lower overall emissions than their <WSD>big brothers</WSD> in the Class 43 IC125 power cars, in the case of CO₂, when you burn a litre of diesel fuel you get 2.8kg of CO₂. <small>[a possible error in the original text]</small> 
Meaning of the word marked with <WSD> tags:  (figurative) Something that is bigger in comparison.