The Su-27 without TVC is very maneuverable that it could challenge F-16C & F/A-18C/E in dogfight. The Su-27 despite being large at 21.9m long, its design is more of a giant kite spreading the fuselage apart from 2 engines. Overall weight is just as heavy as the F-15E and F-14B. Whereas Mig-31, F-111F at 70ft have full volume fuselage making them heavier at 90,000-100,000lb max takeoff weight. There's internal gun in Mig-31 mainly because of requirement to gun down stealth bomber or any high asset value aircraft that it managed to catch up with its high speed should it runs out of missiles (spent on enemy fighters escorting the AWACS, bombers).
The J-20 has more volume of fuselage than the Su-35 therefore the J-20 is considered bigger and heavier. Since stealth aircraft relies on internal bay to retain stealth, China opted for large internal tank and internal bay to carry more missiles and bombs as opposed to US stealth fighters that can't fly multi-role sorties, yes you're right on this. Russian Su-57 adopted YF-23 approach while retaining everything stretch apart wings design to achieve high maneuverability.
I agree, but point is not to belittle any aircraft, is based upon physics, chemestry, that even a high school student should know. To put it simple carbon and aluminium are the lightest materials used in aerospace and all aircraft use them, basically is a limitation by chemestry.
Speed demand different features to an aircraft than stealth or agility.
In 1970 or 2021 if you want speed, no agility and no stealth a shape like Concorde is needed.
If you do not care for speed or stealth just agility, something like Su-29 or AM6 Zero is what you need.
If you want stealth F-117 or have blue is the ideal shape.
What I mean is all aircraft ask contradictory demands, J-20 is no exception, the aircraft is way too heavy, in order to be agile or fast needs lots of thrust.
Add TVC nozzles and it will give a stunning air show demostration, now it only flies like most other aircraft, nothing outstanding.
Su-27 has speed and agility as main contradictory demands but stealth did not play an important requirement.
Su-57 has speed, agility and stealth, like J-20 or F-22,
Su-35 can keep up with F-22 mostly due to better aerodynamics, but F-22 can keep up with Su-35 thanks to raw power I mean very powerful engine.
Speed asks for slender fuselages, basically bullet shaped something like concorde or Tu-160.
Agility basically asks if it is subsonic agility, for straight wings; or LEX and low swept wings like F-18 for something in the region of Mach 2.
Stealth asks for faceted fuselages, flat sides, and diamond shaped cross sections.
J-20 main contradiction is internal weapons bays and s shaped inlet ducts that add substantial weight, volume, size which is reflected on a larger aircraft and larger drag that needs bigger wings and more powerful engines with thrust vectoring.
Thrust vectoring also adds weight too and even can reduce thrust.
Flying wings have troubles of controlability so J-20 retains the main fuselage traights and control surfaces.
In few words contradictions and preference of requirements shape aircraft.
As it stands now J-20 is a bigger F-35 with canards to increase slightly agility.
