Why is the Su-27 so much larger than the MiG-29?
The Su-27 and Mig-29 were the result of high-low concept subscribed by both the USAF and Soviet Air Force (VVS) in the 1970s. Basically, this involved the development of two different fighters, a high performance medium weight fighter crammed with every advanced technology (which made it expensive) and a low cost light weight fighter to make up the numbers. 

In this direction, the USAF announced the FX (Fighter eXperimental) program, which resulted in the McDonnell Douglas F-15 Eagle (designed to counter the misunderstood Mig-25), followed by the LWF (Light Weight Fighter) competition, that gave birth to the General Dynamics F-16 Fighting Falcon.

The Soviet equivalent of these programs was the PFI( Perspektivnyi Frontovoy Istrebitel, Perspective Frontline Fighter), which called for a high performance multirole aircraft to counter the new generation US aircraft and the LFI (Lyogkiy Frontovoy Istrebitel, Light Frontline Fighter), which called for a low cost fighter with respectable ground attack capabilities.

The PFI was required to operate 250-300 km beyond the frontline, with sizeable internal fuel and a comprehensive Communications, navigation, avionics and weapons suite. The LFI, on the other hand was required to operate 100-150 km beyound the frontline, while being easy (read cheap) to build and maintain and capable of using semi-prepared strips while carrying limited armament (initially limited to a couple of medium range AAMs and gun).

In 1972, the VVS tasked the Sukhoi bureau to build the PFI, resulting in the Sukhoi Su-27, while the Mikoyan bureau got the LFI, resulting in Mikoyan MiG-29.



F-15 and F-16; image from sunketh.wordpress.com

Basically, Su-27 is to Mig-29 what F-15 is to F-16; hence the size difference.