I mean, the answer to the question would be a resounding maybe Could be a bigger US, could be a smaller one.
Easy answer would be "The US might push harder for 54-40 to make an additional free state". But that isn't necessarily going to happen - there are plenty of other issues that are involved, and what really is an issue is that the whole free/slave state balance is messed up. I imagine Iowa gets admitted earlier to counter Texas, but that's the biggest change off the top of my head.
The big question is where the western boundary is drawn. The proposed boundaries from the
http://dsl.richmond.edu/historicalatlas/95/a/?view=plate show the rivers following the projected flow to their source up to the Continental divide, which of course didn't exist. However, these are merely proposals, and I honestly feel that it'd continue up to the 100 Degree W line, same as OTL. So Texas would end up being 1/3 of its OTL size, roughly, and this balance could be achieved.
Assuming that is the only change, the westward expansion shouldn't be altered too much. The biggest issue being that the Mexicans won't be agitating for more settlement to distract to Comanches and to try and entrench the frontier, as those would be coming from the USA anyway. So you don't change much except that the population overall might be higher, and that slavery would continue outright in Texas as there would be no abolishment under Mexican law.
Any Mexican-American war that emerges would emerge over California, and in the event of war most of the final borders would probably converge. Just pointing out that the Rio Grande makes a very good border, so in the event of war with Mexico it's possible the the border would be extended south. Also, the Colorado River border means that some of the regions settled by Americans OTL would not at first, so you'd probably have Mexico invite some into that part of Texas anyway - the region between the Nueces and the Colorado, that is. Their population wouldn't be large and a threat to Mexican rule so would likely be accepted - so you could have a small American community emerge there, but not be a majority within the region.
You'd also have a different negotiator than OTL as with a PoD 30 years prior it's unlikely Nathaniel Trist is the one negotiating, and he didn't even press for all of the OTL US's demands. The US would still want San Diego for its port, which did drive part of the expansion south. The original treaty even called for Baja to be annexed, along with any additional territory that could be sought.
Of course, you could end up without an expansionist government at the time, but that's something you could best attribute to butterflies. Same with the timing of the discovery of gold in California. You could have the US try to bite off too much and end up in a war with Britain and Mexico at once, but that is likely doubtful. The war would likely end up a few years later compared to OTL, though with how many Americans moved into California so quickly.
So best case scenario, assuming e
verything else stays similar, Mexico retains the Colorado border, with a straight line connecting the Colorado and Gila Rivers.
Still, it might be better to track Mexico's evolution without Santa Ana being removed from his first period of rule (Assuming he even is elected in the first place, which isn't a bad bet, but still an assumption). He would likely continue his centralization path, combating with the more federalist groups on the fringes of republic. Without the Texas example, there would be far fewer that might actually step right out and attempt to revolt - conversely, this means that many rebellious/secessionist groups may survive longer. This could work either way. You might have a stronger Mexican military that could better fight off the Americans. Conversely, there might be other independence movements - you could have a Republic of the Rio Grande that stretches to the Colorado and has a small American-centered minority north of the Nueces that could end up an influential part of that country.
For its sake, California was already relatively autonomous from Mexico, so Santa Ana's centralization might be viewed with opposition by California proper, and California itself is extremely remote to the Mexican metropole.