Yes AS I thought you are not taking into account the recall back. So you were not doing math properly.
Let say it takes time let call it t to run to the resource this ad a speed of 121.
(I put the cargo disk into the inventory on the way to the resource)
On the way back this will require t*121/85 = 1.42* t
Or with recalling 0*t
So the total time taken for running up and down with a cargo disk is 2.42*t and for calling back and for recall this is just 1*t
So copying for you with cargo disk the carrying capacity is 21692 and without it is 8193
So in time t you would transport with cargo disk 21692/2.42= 8964 and for doing it without cargo disk it is 8193/1 = 8193
So the gain you get is 8964/8193 =1.09 so you carry 9% extra with using your cargo disk.
Same calculation on the worst cargo disk, speed and starting backpack and no pet
Running back and forth with cargo disk will take t+t*65/54 = 2.2
So in time t you are transporting 1150 without and 2650/2.2 = 1204
So you gain (1204/1150 -1)* 100% = 5%
In my case this is with loyalty pet where I started noticing the effect, getting granite out of the mines of Augenhall
1780 normal +5000 pet = 6780 the disk is 1500 and speed normal 75.
To normal in time t is 6780 and with cargo disk 7280/(1+75/64)=3352
So my gain was (3352/6780-1)*100% =-51% and yes I started noticing it that I was quicker without. That started me thinking! And doing the math proper. No my case was quite extreme. But when you have a pet one should take a second thought on efficienty.
Also now I have proven my formula just by going through it in saperate way.
I will check with a stop watch when I have found a mother load on some distance from a machine. I'm not that different from what I was with the granite.