Synchronous Generators Parallel with different capacity on Island Mode

Hello Hassan here,
I am working in a power plant where we have two generators of different capacity.i.e one of 10MW and the other is of 5MW. When they are operated on island mode and at auto condition. 5MW genearator would start to loose its reactive power and shift it to other generator.Why it is happening?.Does we need any AVR tuning or something?