B737-800W burns about 25 kg of fuel per hour per extra 1000 kg.
Ryanair went from cloth seats to leather seats, which don't recline any longer (and probably have a lighter frame as a result of that).
This resulted in a weight increase of 500 kg for an aircraft so fitted out.
500 kg extra means about 12 kg of fuel extra per flight hour.
However, this is offset by lower cleaning and maintenance costs. Leather can just be wiped clean, cloth seats need deep cleaning if somebody is sick.
Knowing Ryanair, they have carefully calculated the effect of the higher weight and decided that it is paid for by lower running costs.
Weight alone is not the decisive factor in these matters. 'Cost of ownership' or whatever you want to call it is.