Why are people saying lower cp mons will be better for gyms?

It is just speculation, with nothing to back it up - we will know all soon enough.

Just chill. It's still all speculation based on current available info. Just wait until the gyms are unlocked and see for yourself.

Don't worry. Blisseys will still be the hardest defender to take down. Lower CP mons will still be cake to defeat.

Someone speculated that the lower CP mon you use to battle might reduce the defending pokes motivation more. Similar to prestiging a gym. But that was 100% speculation.

The picture that person showed was the poke losing CP not motivation.

The poke gets returned to the owner when motivation gets knocked to zero not CP.

A stronger more powerful mon like a Blissey will still be better than some 1200CP chansey and it won't even be close.

Why are people saying lower cp mons will be better for gyms?

They try to interpret the pieces of information given until now:
If a pokemon loses motivation as a given percentage of its CP, the absolute loss is bigger for bigger mons. When assuming a berry gives absolute amount of motivation back, maintainance for high-CP-mons would be more costly.

As it looks now, higher level/cp pokemon will probably lose motivation quicker than lower cp mons. If that is true, suddenly defenders like Umbreon and Steelix etc, will be as viable as defenders as for example a Blissey.

I guess, they mean that there are some mons with low CP but they are very tanky so they will be harder to defeat. For example, Umbreon, Steelix, Miltank etc. Such mons will surely be better for gyms than all these Rhydons, Gyaradoses who takes seconds to defeat.

The only thing that makes sense to me, is Motivation declined by a certain amount by how fast or how many pokemon it takes to defeat it. And increases with how many pokemon it defeats. But i have not seen anything on it being increased without berries. Just makes sense to me.

Nobody has any idea right now whether gyms will be defendable under the new system. If any single player still can clear even the strongest gym, and with CP-ranking gone, then it does not matter what is in the gym. People will just place throw-aways. But if good defenders have a chance at holding a gym, certainly a high-level mon will do better than an otherwise identical low-level one.

Nobody even knows how the Motivation-meter is set up.
I mean.. Maybe higher CP mons lose a certain amount of Motivation faster, but who knows if all Mons have the same amount of Motivation..?

It's just: relax, have a cup of tea and wait.

For me it's probably a sad story, since in my rural cowlike village Nobody plays. Maybe some 10 year olds.. but never would someone 20+ Go and have a raid with ne :(
Poor me

Official announcement contained this line:

"To help balance different Pokémon strengths and abilities, stronger Pokémon generally lose motivation more quickly than those that are not as strong."

It's unclear from that, however, what "stronger" means. It could be based on level, CP, or species (possibly with "tiers" of species - they've already sorted the species into tiers for the raid bosses).

If it's CP or level, that gives an incentive not to power up your mons, which would be weird; weird enough that I doubt that's what it's based on.

At best, I think the motivation system can help balance the difference between high cp and low cp mons. I still think your level 30+ defenders are going to be very useful for gyms. Higher level = more damage dealt in battle + more hp to make battles last longer. Maybe lower cp mons will lose less motivation, but they'll be a lot easier to plow through in battle, which means fewer potions required form attackers.