Meaning of "bulging with Pongo's lunch" in the following sentence
Hearth-rug
Bulging with lunch
Two pennies worth
Clam a stolid or closemouthed person
On the occasion to which I am referring, Pongo's Uncle Fred—Lord Ickenham— , stood pink and genial on Pongo's small carpet in front of the fireplace, with a big belly full of Pongo's lunch and covered in the smoke of one of Pongo's cigars, and said: 'And now, my boy, for a pleasant and instructive afternoon,'
When the uncle said this, you will easily understand why (Pongo), the unfortunate young suddenly quiet man, looked at his uncle as he (Pongo) would have looked at a small stick of dynamite lighting up in his presence.
You have understood each and every word literally in the passage. The meaning and intent of the paragraph is not really about English but about the metaphorical or cultural uses.
"hearth-rug": literally, a rug at the hearth/fireplace. Metaphorically, a place to sit and relax after lunch
"bulging": literally, expanding in a spherical manner. Metaphorically, he ate a lot and his stomach feels extended.
"clam": literally, a sea creature. Metaphorically, a very 'public-school' informal manner of referring to a young man "old chap", "old bean" etc.
"two penn'orth": literally, a very small amount of change. Metaphorically, a trivial amount, but in distinction to dynamite which follows a bit incongruous (for humor's sake).
So you have the literal meaning of everything, you just have to slide over into understanding things not so literally and with a view to humor.
The idea is that the person saying "and now, my boy, for a pleasant and instructive afternoon" is full to bursting from lunch, and relaxing with a fine cigar. Both of these states would normally, in English-speaking high culture, indicate a person who would normally spend their afternoon napping, not teaching. So, "you will readily understand why the unfortunate young clam" - "clam" here being some deprecative term for a junior person and/or someone staring impolitely - "gazed at him as he would have gazed at two-penn'orth of dynamite, had he discovered it lighting up in his presence" - in more modern parlance "gazed at him as he would have gazed at an M-80, had he seen the fuse being lit in front of him".