Reason for limited engine fuselage on Sukhoi fighter aircraft?
In a fighter you generally want to optimize fuel and ordinance capacity as well as have competitive performance. This use case generally leads to a design that is different than your average transport category aircraft. The SU-27 has some similar characteristics to the F-14 Tomcat and it should be noted


  The aircraft had a large wing, clipped, with two separate podded
  engines and a twin tail. The ‘tunnel’ between the two engines, as on
  the F-14 Tomcat, acts both as an additional lifting surface and hides
  armament from radar.


Here is a good picture of the engines from the back 


(source)

You can see that they could connect the engines but that would increase cross sectional area, disturb the center portion which helps add lift and add weight. If there is nothing they need to stick in there, there is no reason to have such an area on the aircraft. Its also worth noting this is not terribly different from the back of an F-14.

There is not necessarily a whole lot of metal surrounding any aircraft engine. 

In the case of the SU-27 it also looks like the engine coverings are unpainted, this may be for heat related reasons.