Because the ARP rod bolt exerts more clamping force on the rod, the shape of the big end hole min the rod may change. Also Factory A series bolts are a finger tight push in, where as the ARP need to be pressed in. The force of pressing them in can also alter the shape. it's not a huge expense to have an engine shop resize them for you. Having your rods resized is part of a sound performance engine build anyway.

ARP A-series rod bolts do not need to be pushed in, they slide in with just finger force just like the stockers.

While resizing is the correct thing to do, I have never needed to resize an A-series rod after fitting ARP's when the ARP's are torqued to the correct settings, all of them have stayed perfectly in spec (unlike SR20 and RB's that can put the big end way out of round) Either way you need to measure as every motor is slightly different.

You guys must please explain to me more about this re-sizing story. I would have thought you just torque and you are done.

Also, I don't understand how the ARP can increase clamping force in the con rod. On the VW site and I think somewhere on this site I read that the ARP bolts have a lower torque setting than standard rod bolts.

Dont know what stock torque specs are but ARP recomend useing the strech method, streching them to .0051 +/- .0002.

But It says if you dont hav a stretch guage to torque to 28ft lbs, then to loosen entirely, repeat then tighten for a third and final time also to 28ft lbs

ARP often use a lower torque value than stock because the thread is finer (which still means it can actualy have a higher clamp load)

Think of a con rod and how the big end cap bolts to the rod, the more clamping force you give it the more out of round the circle will become and thus you need to then resize the rod. What I said is the A-series rods are quite resiliant to going out of round when adding a little extra clamp load by using ARP (some rods can go way out of spec)

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Thats what I thought untill I tried to fit mine. They had to be carefully driven it. (I didn't have a press handy at the time) The old bolts were a free and easy finger tight slide in fit. (Actually they were free-er that I would have ideally liked) So It wasn't just that the rod bolt holes in my rods were tight.
The instructions that came with the bolts (albiet that they are most likley a generic instruction) said after fitting new bolts, the end of the rod would need to be resized.

Resizing rod big ends [also known as 'close & hone'] is a standard reconditioning procedure, & has been since the invention of the slipper big end bearing about 100 years ago.
It assures that the rod big end is perfectly round & at a true 90 degrees to the vertical centerline of the rod when in the state in which it will be used, that being, clamped up to the rated torque.

It is commonly done to rods where the bearing has spun in the rod & as long as the rod is otherwise undamaged, it is an economical way to restore a rod to service.
This procedure is sometimes used to ensure that all rods are exactly the same length, C to C, in a blueprinted race engine.

It is not some black art practiced by highly paid engine scientists in the deep recesses of a secret race engine laboratory belonging to some multi million dollar factory backed race team.