"The others deferred to Congress because at the time, Congress was the important part of government. "
Oh I absolutely agree. Which is why I give credit to Polk for breaking with the milieu he was in. It took at least a modicum of courage to do so. The Whigs in particular were staunchly committed to Congressional supremacy and Party government. Jacksonian Democrats less so.
"Polk didn't do anything insanely significant. He got territory, and he helped the country, but he didn't do anything close to what, say, Lincoln did. He did as any President should, though "
I'm not saying he was a Lincoln (although had Lincoln did lasting damage to the power of the Presidency, it didn't recover until T.R). But you have to compare him to the Presidents of the era - Harrison (who died, not his fault), Tyler, Fillmore, Pierce, Buchanan, Taylor, even Van Buren & Jackson. He was superior to them all. Jackson, to his credit, was someone who tried to advance a new meaning of the Presidency. But he also ushered in the spoils system - a problem which preoccupied the country until well into the 20th century, and presided over the elimination of the National Bank. So I'd take Polk over Jackson also.
Now, Polk's achievements were not limited to his military success, a military campaign which was flawlessly carried out, I might add (I say this as someone who acknowledges the campaign was less than just). He also thoroughly defeated the Whigs on the tariff issue and internal improvements issue, which might not seem like much to us now but this was perhaps the singular most important issue of 19th century politics aside from the slavery issue of the Antebellum period. One of the reasons why he (and Jackson) were so hated by his opponents was because of their willingness to use the veto against bills for political reasons. The tradition had been only to use it if the President thought the bill was unconstitutional.