Yes, and so we have a situation where 10 ton and up are made with one type of structural philosophy but 10 ton and smaller machines are made differently. We know that now. The question then becomes, "Why?"
I don't know the answer for sure, but can hazard a theoretical guess based on a lifetime of engineering. My guess is that they differ because this is not a linear design problem, and that ten tons is approximently where the required yield strength vs load/size slope begins to rise very rapidly.
It all comes down to the cost per tonne of material moved. Under 10 ton are compact machinery, mostly used to be available when needed, which isnt 24/7, not even 8/5
Tracked excavators up to 20 ton usually have a design life up to 15.000hrs, up to 90 ton generally 20.000hrs, and above that, mining machines are designed to run 30 to 35.000hrs between rebuilds. That translates into Cat C32 engines putting out just 1000hp in an excavator: In a 90 ton excavator you will find a 750hp 18 liter.
Generally speaking, there are three design grades:
Compact grade up to 10 ton: Designed for 8-10.000hrs operating life. Usually used for odd jobs around the worksite, their performance is evaluated in an abstract way, they have to be a jack of all trade and usually carry materials around the jobsite more than they do excavation in ton/hr
Construction grade up to 90 ton: designed for 15-18.000 hours operating life. These machines performance is measured in ton/hr but they also finish grade a jobsite.
Mining grade 120 to 900 ton: Designed for 30 to 35.000hrs between rebuilds. Front shovels are usually retired after 80,000hrs. A whole fleet of haulers depends on them, so uptime is key, no time to replace pins and bushings every 3000hrs, which in 3 shift mining means at least twice a year. They dont do jobsite cleanup, they are too expensive for that work because a whole mining operation depends on them. They only do full buckets, year after year.
There might be different design lifes used for 800kg mini diggers than for an 8 ton midi digger, i dont know because ive never worked on anything smaller than 10 ton. I can just tell, in the 11 to 18 ton wheel loaders weve allways put 42CrMo4 hardened and then ground pins, with nitrated hardened bushings with cross cut grease grooves, and when i started at my current job they had the same supplier of bearing bushings, we do modifications and special booms (high reach demolition, long reach, material handling) on excavators of 20 to 200 ton.
Whenever the operating hours per year increase, it pays to use higher quality pins and bushings. Generally TLB's, compact loaders under 10 ton, and small excavators dont run 1500 operating hours per year, and when they do, they usually carry stuff around the jobsite, not pushed hard because there is a line of haul trucks waiting.
Whenever the hourly cost of the machine goes up, so does the price of downtime, and the cost of longlife parts drops in relation to the total build price of the machine.
