Ye are not straitened in us That is, "It is owing to your own fault that you are not able to share in this feeling of cheerfulness, which I entertain towards you. My mouth is opened, so that I deal familiarly with you, my very heart would willingly pour itself forth, [1] but you shut up your bowels." He means to say, that it is owing to their corrupt judgment, that the things that he utters are not relished by them.

Footnotes

1 - "Mon coeur mesme s'ouuriroit volontiers pour vous mettre deuant les yeux l'affection que i' ay enuers vous;" -- "My very heart would willingly open itself up, so as to place before your eyes the affection which I entertain towards you."

Ye are not straitened in us - That is, you do not possess a narrow or contracted place in our affections. We love you fully, ardently, and are ready to do all that can be done for your welfare. There is no lack of room in our affections toward you. It is not narrow, confined, pent up. It is ample and free.
But ye are straitened in your own bowels - That is, in the affections of your hearts. The word used here (σπλάγχνα splangchna) commonly means in the Bible the tender affections. The Greek word properly denotes the upper viscera; the heart, the lungs, the liver. It is applied by Greek writers to denote those parts of victims which were eaten during or after the sacrifice - Robinson (Lexicon). Hence, it is applied to the heart, as the seat of the emotions and passions; and especially the gentler emotions, the tender affections, compassion, pity, love, etc. Our word "bowels" is applied usually to the lower viscera, and by no means expresses the idea of the word which is used in Greek. The idea here is, that they were straitened, or were confined in their affections for him. It is the language of reproof, meaning that he had not received from them the demonstrations of attachment which he had a right to expect, and which was a fair and proportionate return for the love bestowed on them. Probably he refers to the fact that they had formed parties; had admitted false teachers; and had not received his instructions as implicitly and as kindly as they ought to have done.

Ye are not straitened in us - That is, Ye have not a narrow place in our affections: the metaphor here is taken from the case of a person pent up in a small or narrow place, where there is scarcely room to breathe.
Ye are straitened in your own bowels - I have not the same place in your affections which you have in mine. The bowels are used in Scripture to denote the most tender affections. See the note on Matthew 9:36.

Ye are not (h) straitened in us, but ye are straitened in your own (i) bowels.
(h) You are in my heart as in a house, and that no narrow or confined house, for I have opened my whole heart to you; but you are inwardly narrow towards me.
(i) After the manner of the Hebrews, he calls those tender affections which rest in the heart, "bowels".

Ye are not straitened in us,.... The meaning of which is, either you are not brought into straits and difficulties by us; we do not afflict and distress you, or fill you with anguish and trouble;
but ye are straitened in your own bowels; you are distressed by some among yourselves, who ought not to be with you, with whom you should have no fellowship and communion: or thus, you have room enough in our hearts, our hearts are so enlarged with love to you, that they are large enough to hold you all; an expression, setting forth the exceeding great love, and strong affection the apostle bore to the Corinthians; when, on the other hand, they had but very little love to him comparatively; he had a heart to hold them all without being straitened for room; and among all them they could scarce find room enough in their hearts and affections for him.

Any constraint ye feel towards me, or narrowness of heart, is not from want of largeness of heart on my part towards you, but from want of it on your part towards me.
bowels--that is, affections (compare 2-Corinthians 12:15).
not straitened in us--that is, for want of room in our hearts to take you in.

Ye are not straitened in us - Our heart is wide enough to receive you all. But ye are straitened in your own bowels - Your hearts are shut up, and so not capable of the blessings ye might enjoy.