Decaying more and more,Till he becameMost poor: (3-5)
Life outside of Eden is just one bad thing after another. The accumulated badness transforms Adam from a just-created dude with everything to a poor and miserable man. "Decaying" sounds more like corpses on Halloween than hoppy bunnies on Easter, driving home the negative transformation that Adam is undergoing.
With theeO let me rise (6-7)
The speaker begs God to let him rise out of Adam's sin and transform himself from a miserable sinner into an adoring Christian. And hold on to your iambs, we've got a parallel transformation in line length. Notice how the shape of the poem mirrors the changing content. When Adam and the speaker become thin, poor, and despairing, the stanza also contracts, from 10-syllable to 2-syllable lines. And when the speaker's on the upswing, feeling great and soaring high, the lines get correspondingly bigger, from 2 back up to 10 syllables.
Thou didst so punish sin,That I becameMost thin (13-15)
Borrowing "became" from line 4 in the first stanza, Herbert reemphasizes his bad-to-worse transformation. Being born with Adam's inherited sin is bad enough, but once God gets in on the game, punishing him with "sicknesses and shame" (12), Herbert wastes away. Line 15 marks the low point, in both Herbert's life and line length: Herbert is "most thin" and things aren't looking good.