aircraft having internal weapons have more internal volume to accommodate the weapons, the racks and the systems to launch its weapons, thus will increase their internal volume, see how much bigger is F-35 compared to F-16 or Mirage III compared to F-106, in order to have similar performance you need much more powerful engines.How can internal weapon bays increase drag of the 5th gen stealth jet kindly explain maybe it increases little bit weight and both Russia and China working their 5th gen engine for their 5th gen jets from day one both Su-57/J-20 known to have interim engine,and as they(China/Russia)have different philosophy then USA they will use tactics against enemy(USA)shoot and scoot (leave)guideness of BVR is/will be the job of AWACS @Su33KUB
In F-35 it has a wider Cross section than F-16 with almost same length, but F-106 is not only wider than Mirage III, but also longer
As the F-117 had shown, stealth affects every aspect of a design. Internal weapon carriage, a must for a stealthy design, increases the cross section of an airplane. Larger cross sections increase supersonic drag and work against supercruise. "A stealthy airplane requires a big weapon bay," explains Hardy. "And the landing gear and the inlet duct want to be in the same place as the weapon bay. You wind up with a guppy that won't go supersonic unless you make it very long with huge engines. Such an approach is a nonstarter because the airplane would be way too expensive."
Maneuverability requirements tend to increase the size of the wings and tails and make the engines bigger than necessary for supercruise alone, all of which make stealth more difficult to achieve. Those few pilots who were briefed on the F-117 knew about compromises in speed, maneuverability, payload, and other capabilities that went along with an all-out approach to stealth. Fighter pilots who would be flying the ATF would not willingly sacrifice these capabilities for stealth.
https://www.codeonemagazine.com/f22_article.html?item_id=179
see J-20 has a huge fuselage because also has a huge cross section, as the article says
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