A few semi random observations here:
1. The reserves could expand quite rapidly into a large force if we simply look at the issue of available bodies. A typical example was a Reserve CoOp course I was 2I/C on a few years ago, which was limited to an intake of 30 for London and surrounding area. One of the school councillors responsible for their school's intake told me that over 100 students applied from that school alone. Most of the other councillors had similar observations, meaning that up to 1000 recruits could have been taken (although after weeding out I suspect the real number would have been a still respectable 500+). I will also note that the CoOp program is NOT heavy promoted or advertised in the schools (often because *we* don't even know until the last minute if it will be given the green light or not: the mad scramble for course stores and training aids is something to behold), and in many schools the administration is indifferent or even hostile. In this case the limiting factor is more the size of the instructor cadre and amount of uniforms and training stores and areas that can be secured.
2. I think the real locus of evolution needs to be in the Regular Force. Looking at conflicts like Ukraine, Gaza or the general mess in the Middle East, the types of forces which are being used tend towards SoF supported by a wide range of enablers (Cyber, PSYOPS, ISTAR and remote fire support in the form of UAVs and aircraft etc.). Large numbers of men are needed for "holding" actions rather than fighting (the bulk of the Russian forces are there on the border seemingly to mesmerize Ukrainian and NATO decision makers, the fighting was done by irregulars backed up with SPETSNAZ troops and training until quite recently. Even now, the numbers of Russian troops "on vacation" in Ukraine is a fraction of the potential available). These sorts of troops require intensive training to receive the proper skill sets, and to practice them often, hence the need for full time and relatively long term troops for these tasks.
3. COIN, STABOPS and other forms of "holding actions" (the Russians don't have to be the only ones massed on the border, after all) needs lots of manpower, but fewer of the specialist skills and training of SoF and enablers. Bulking up the Reserves to do the traditional "peer to peer" type fighting and provide the boots on the ground for other manpower intensive missions makes sense because these missions are shorter term and much less frequent. Of course, lots of manpower still translates into lots of equipment, from actual boots (I'm down to one pair again, so the issue rankles) to trucks, AFVs, aircraft and ships, plus the stores to train people and run things. Now while this is expensive, it isn't impossible. Nordic nations have entire mechanized battalions "under tarps" waiting for their soldiers to arrive for their two week annual training exercise. US National Guardsmen only parade one weekend a month, yet seem able to operate in mechanized formations as well. We should look at how they can pull that off with far less parading then *we* get.
This turns a lot of the arguments on their heads, but the reality we are facing is quite different than even the end of the cold war, when I first learned about fighting a large peer enemy force, much less the 90's or '00's.