Each of the islands (allods) has its own magic and its own laws of physics. In other words, on each server – magic, substances and materials of weapons and armor (steel, mithril, adamantine, etc.) works differently. Anything above a certain material for the specified allod has extra weight or greatly reduced characteristics.

In other words: things for higher difficulties work much worse on lower difficulties.

The table shows which material is relevant on which allod; the material that is penalized is shown in brackets:

Allods (server) number Material
#1 easy iron (steel +weight)
#2 medium steel (mithril +weight)
#3 medium mithril (adamant +weight)
#4 medium adamant (meteor +weight)
#5 hard meteor (crystal +weight)
#6 horror+ crystal

Note: chain and open armor weighs significantly less than full armor.

Thus, the crystal equipment on the #5 “hard” weighs 3 times more than on the “horror”, at the same time on the #3 “medium” the crystal is practically useless (defense, armor, damage – like at iron). Additional examples:
At #1 “easy” steel weighs ~3 times more; anything higher than mithril is worse than iron.
At #2 mithril weighs 6 times more; anything higher than adamant is worse than iron, etc.

Also… Leather armor has increased protection at server #1/2, although this is not written in the description of a particular item (check your character sheet).