I responded to this notion with the old Creationist tactic that says scientific evidence doesn't count unless you were there, in an effort to get you to understand the fallacy of your statement...
Which is bullshit, btw. Apart from the fact that you're confusing phenomenology with objectivity (and therefore really DON'T have any idea what it "feels like" to be a mouse or how a small animal actually interprets the world or experiences its own inputs) you're basically assuming that locutus needs to be enlightened and shown the error of his ways by your wise and clever ruse. Unfortunately that's just an ultra-smug version of the strawman fallacy and it doesn't actually demonstrate anything.
That being, you made the direct claim that one cannot understand an object unless one has experienced being that object.
Which is a well-known philosophical position at least a thousand years older than the country you're currently living in. There is a philosophical rift here in which reductionists assume that that any phenomenon can be quantified and quantification of a phenomenon is equivalent to experience; then there are those who assert that the experience of a phenomenon is distinct from the description of a phenomenon in important ways, and that you cannot actually know the experience of a thing merely by having it described for you. I lean towards the latter category, primarily because getting people to move from description to experience is a huge part of how I make my living and I know only too well that these things are totally different. Not just between any two species, but between any two PEOPLE.
In short, you're making an indirect philosophical argument based on no evidence and then smugly mocking locutus for not following your logic. There isn't really a point to be made here except that you should probably calm down and think through what might be wrong with your original assertion: that there's no way to confirm that what you imagine an experience to be like will actually be similar to the real thing.
Circling back a bit, I find a couple of things. Firstly, brain SIZE doesn't have a lot to do with intelligence at all, it's mostly about topography. You can actually pack a lot of brain tissue into a very small space depending on how you fold white and grey matter inside the skull. The thing is, a more tightly packed brain may be more susceptible to injury, so there might be an evolutionary advantage in letting the brain cavity grow in proportion to the size of the creature's skull as opposed to keeping it tightly bound.
OTOH, it's worth pointing out that brains DO NOT operate like computers (modern analogies notwithstanding) and do not "process information." They react to stimuli in deterministic ways, and their reactions affect subsequent reactions later on, but memory and information are not stored within the brain, only expressed in various ways to the extent the pattern of expression is repeatable. Human intelligence (such as it is) works the way it does because we excel at abstract thinking; this makes us accomplished generalists, which is good for us because it makes is highly adaptable. On the other hand it means we basically suck at just about everything and have to make up the difference with technology, where as other life forms are much better at identifying sounds, objects, special relationships, movement patterns, kinesthetic patterns, and assessing threats in their immediate environment. In other words, most life forms appear "less intelligent" because they devote most of their thinking power to simply staying alive and/or not getting eaten. Humans appear more intelligent because we devote an anomalous amount of brain power to all kinds of meaningless bullshit like cat videos, presidential elections, and arguing with people on the internet.
