So ive just noticed very small cracks going around the side wall of my tyre. Theyre schwalbe marathons. I useually pump them up between 80 and 100psi depending on how the ride feels. Heres a picture of it. ive circled the cracks with a red line. Could this be due to too much psi in the tubes?

Rubber often develops cracks due to the influence of ozone and UV radiation. The strength of a tire is in the fabric plies, not the rubber so superficial cracking like your photo shows is common and expected. Unless chunks start coming off (unlikely) you shouldn't worry.

I worry with some cracked tires, but those cracks are small enough not to worry me.

thanks very much for the reply. the only reason i was worried was because they only show up when i pump up my tyres, under the stress of being stretched by the inner tube.

yes, not to worry. As noted the rubber dries and develops this check cracking over time, because of tire flex as you roll. It's meaningless as long as the tire's fabric plies are intact, as evidenced by even width without lumps or bumps when full.

Minor comment, the tube doesn't stretch the tire. The air inside does, pushing on the tube and tire wall together. The fabric has some give the same way a rope or even a steel cable will stretcha bit when tensioned, which is why the cracks are more visible when the tire id pumped up hard.

You are mostly correct with two minor(ish) nitpicks. First, rubber doesn't "dry out". There is nothing in the isoprene polymer that is liquid and can be removed by evaporation. Tires can weather check but that's not the same as "drying out". That is oxidation of the rubber causing it to lose its elasticity. The lower elasticity combined with the hysteresis of loading and unloading the tire constantly leads to cracking.

Second, the difference in dimensions between a tire that is inflated and a tire that isn't is really small as long as both are unloaded. The tire will stretch a little under pressure but it is a minor change. Certainly not enough to cause the rubber to crack. The cracking is all due to ozone induced oxidation and sidewall flexing.