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Thrust weight ratio is not measured by unit, it's total amount of thrust generated by the engine so a larger engine is a lot easier to develop certain amount of thrust since weight does not gain proportional to size since while you can create a larger cavity for combustion to increase thrust.
No, it is much harder for larger engines to develop the same TWR as significantly smaller engines.
Strictly speaking neither thrust nor structural weight increase proportionally to volume, but roughly speaking their rate of increase is closer to the rate of increase in volume, rather than to area or length. In addition, as size increases, structural weight increases at a higher rate than the increase in volume or thrust (assuming similar materials, design skills, and engine type).
Your assumption that "weight does not gain proportional to size since while you can create a larger cavity......" is wrong because if you use the same thickness of materials to make larger fan blades, axles or casing, your engine will quickly fall apart. This is especially true for fan blades because you're dealing with not just the material's own weight but also an extremely strong centrifugal force proportional to its radius.
Even for static parts, an increase in size means more material strength will be used to support the structure's own weight, therefore, if you use the same material, the total volume of solid parts will need to increase faster than the internal volume of the engine.
This is the same reason why you can find insects or even birds with extremely long and slender legs, but rhinos and elephants can only walk with thick and stubby legs. If you enlarge slender leg animals to the size of an elephant they will immediately collapse. An ant can have an extremely high "TWR" (how much it can lift in proportion to its bodyweight) compared to an elephant, that doesn't mean the ant was "designed" more intelligently than the elephant. It was just the law of physics.
Besides weight, you're also dealing with the challenge in material tolerance and fabrication. It is relatively easy to fabricate small parts in exotic materials (e.g. single crystal alloys), but it is significantly more difficult and sometimes exponentially more expensive to make the same part twice the size while still meeting performance and durability requirements. Controlling fracture and creep deformation becomes harder to do as you scale up, as well as designing tolerances for thermal expansion. Sometimes this become impossible and you'll have to develop even more exotic materials, increase the number of parts, and change the overall design.
Let me reiterate what I said before, in case somebody didn't see it at the end of last page:
You can't directly compare the "advanceness" or "generation" of engines in different thrust classes just by looking at their TWR numbers.
For example, the J85 with afterburner can reach a TWR between 7-8, and the F-100/110 with afterburner can also achieve a TWR between 7-8, but they're not really the same generation of engines, and they're not "equally advanced" or "equally difficult to develop". There are also special use turbojet engines than can reach insane TWR, such as the RD-41 that the Russians produced back in the 1980s, it has a TWR of 14 or above. You can push the TWR way up by greatly reducing overall engine size or by reducing durability, the TWR number alone is not indicative of how advanced an engine is. Only when comparing engines in similar thrust class and usage can you use TWR to compare how advanced the engine is. Even then, TWR is not the only determining factor, just a quick indicator.
