It would actually be significantly smaller, Texas petitioned for annexation in part to get America's backing of their full territorial claims against Mexico, without that millitary force Texas's millitary planning would be entirely defensive to avert reconquest or conversely to avoid the Americans just taking them by force if they ever pissed the big guns off for some reason.
If an alternative Mexican American war erupted the end result would revolve around Texas serving as a buffer state with other borders being set at difficult to traverse natural boundaries.