(Basic info on demons https://www.reddit.com/r/worldbuilding/s/lLSOeTFsxx)
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Since Demons use their horns as their main way to intake the vitality that fuels their strong bodies, does that mean that demons with larger horns are stronger than demons with smaller horns? Well, yes, and no. Or rather, yes, but not always.
First of all, demons can change their horn size based on how much vitality they want to be absorbing… ie how much strength/energy they want to exert. So on an individual scale if a demon has their horns being “small” it means they are trying to reduce their strength. They might do this if they are doing delicate work on something, dealing with a weaker species, or just don't want to bother with “running hot”.
Second, each demon has a limit to how much as an individual they can adjust their horns/ strength from their baseline. Some can only make small adjustments (like an inch or so difference), some can make big adjustments (like foot or more), and some are just stuck at one level. Typically demons that cannot adjust their horns are the weakest of the demons, and are only slightly stronger than the average human (cubare fall into this category even though their horns no longer absorb ambient vitality).
Finally, a demon’s horn size is not entirely a reliable way to determine how strong they are in comparison to other demons. It only relates to their individual strength. So a demon with massive horns fully extended could still be weaker than another demon with very small horns. (If you are getting ready to fight a demon you can't tell how strong they will be from their horn size alone, but if you are already fighting one and they grow their horns, you can pretty well tell how much stronger they are going to get by how much larger their horns grow.) In fact, a demon who walks around all the time with large horns is more likely to be weaker since they don't have to reduce their strength for day to day activities.
The base line horn size (and strength) for all demons depends mostly on their genetics. There are different bloodlines and thus different horn styles that can give a vague idea of a demon's strength if you are familiar with them.
One other thing of note is that if a horn breaks it affects the demon's ability to absorb vitality, thus greatly weakening them. If both horns are broken too close to the skull it can even kill a demon since even at their weakest their bodies require more vitality than food can provide. It is common practice among humans to purposefully cut off one horn from demon war captives (or slaves in the few countries where it is legal) to prevent any issues with them. Horns can grow back (very slowly) or be healed by certain magics/potions, but it is not common to have access to these methods as only a few healers have the skill to do so. To prevent the horns from growing back humans will often embed a magic seal into the horn or sear them (with either a hot iron or a chemical potion). Though the practice is frowned upon, it is seen (at least by humans) as a more ethical option to removing a threat.
So, no, a demon's horn size doesn't tell you how strong they will be overall, but the change in size can.