Thought is a process. It's dynamic. It's an emergent quality related to system complexity. It's software running on the hardware substrate called the brain and nervous system. It's a particular* type of pattern deployed across networked neurons, neural networks if you prefer. Thought is not just the neurons; it's the neurons and the pattern. It's not just the wiring; it's the wiring and that which flows through/across the wiring. It's not just the firing; it's the firing and the complexity and pattern of the firing.
The bigger the substrate, the greater the number and complexity of patterns that can be played out. The more RAM available, the greater the number of applications that can run simultaneously, and the larger the size and complexity of the applications that can run at all in the first place. Likewise the relationship between brain and thought. Size, or strictly speaking, volume, or more strictly speaking, density, does matter. It matters in terms of whether there is space available for wiring and re-wiring to occur. A smaller brain can and frequently does mean smaller thoughts, but not necessarily so. Damage to a particular portion of brain does not necessarily mean permanent damage or loss of cognitive function. There is often scope for the restoration of temporarily lost function by virtue of a non-damaged area of brain becoming or being made available as the substrate on which the pattern of that lost function can be played out.
There is a large amount of redundancy built into the brain, redundant matter. As evidenced in the cases of people experiencing cognitive deficits following major brain injuries who go on to regain lost cognitive function wholly or partially. Strictly speaking, it's not about redundancy; it's about potential. It's about that which is available to be used but which is not currently in use. A motor vehicle's spare wheel is not redundant; it's just not in use (and when it is in use, it's no longer "spare").
Isn't it great? To have a weird mental electrician able to “re-wire” the brain to provide the appropriate substrate on which to run certain patterns. Brain damage can and does have an impact on cognition, but in many cases the impact is temporary, lasting only until the electrician completes zir job.
Of course, if you have no brain at all, no patterns can form or be formed. And if you have a very little brain, a very little number of patterns can form or be formed.
*exhibiting regressive self-feedback and feedback-response characteristics.