The one with the electrode holder looks to be about 1/16" smaller - if insulation's the same thickness on both, that'd mean the smaller one's either a slightly thin 1/0, or a slightly fat 1 gauge - here's a chart
American wire gauge - Wikipedia
hit the page down 5 or 6 times for the chart - ignore everything but the left side, shows gauge (AWG) and diameters.
Here's the heavy math

- 1/16" is just over .062 inches, so if you take the 2/0 diameter and subtract the 1 ga. diameter
0.36480
- 0.28930
= 0.0755
Which is more difference than your pic show; so your smaller wire could be either 1/0 or 1
You could also do what I did with my "too short/too small" original lead - add some tweco style connectors (male/female) and use the original lead as a whip, then just get something like these -
Access Denied
If you buy just black cable, you can always "color code" the ends, tweco and others come in male/female pairs in either black or red. Here's what I did -

there's a total of 101 feet of cable; some red, some black, electrons don't care so I can mix/match whatever reaches - I also put the same connectors on my mig grounds, so if needed I can have any ground up to about 80 feet, and the 30A spool gun lets me reach up to 30 feet from the welder and ground it anywhere I want...
I also bought some DInse connectors that fit the new little tig/stick machine and built some 1 foot adapter cables with the tweco types on the other end, so the little machine can reach anything too -
Here's what I got -
US Forge Welding Black Cable Connectors #4 thru #1/ - - Amazon.com
US Forge Welding Red Cable Connectors for No. 4 thru No. 1/ - Power Welders - Amazon.com
And the Dinse to make adapters for the little tig/stick -
WeldingCity Welding Cable Quick Connector Pair DINSE-Style 3Amp-4Amp (#1-2/) 5-7 SQ-MM - Mig Welding Equipment - Amazon.com
I got my raw cable at the LWS, they usually treat me good (since 1979) - all my cables are 1/0, 100 foot "round trip) is no problem running 5/32 7018, and that smaller diameter electrode "whip" lets me have flexibility where I need it WITHOUT much loss in the cables -
If you're gonna make more than a couple of your own cables, do yourself a favor and get one of these
https://www.harborfreight.com/10-in-cable-cutters-61422.html
Much easier cutting big wires, doesn't "squish 'em out" near as bad.
Also, make sure any connectors you get will handle whatever size cable you get; if they say "up to 1/0, you will NOT get a 2/0 cable in there unless you wanna cut a bunch of strands off it... Steve