秤鉈壓斤 Small but of Substance
行者笑道：「你原來沒眼色，認不得人。俗語云：『尿泡雖大無斤兩，秤鉈雖小壓千斤。』他們相貌，空大無用：走路抗風，穿衣費布，種火心空，頂門腰軟，吃食無功。咱老孫小自小，斤節。」行者笑道：「你原来没眼色，认不得人。俗语云：『尿泡虽大无斤两，秤铊虽小压千斤。』他们相貌，空大无用：走路抗风，穿衣费布，种火心空，顶门腰软，吃食无功。咱老孙小自小，斤节。」
逐句解 Line by line
Pilgrim said with a laugh, "You have no eye for men, it seems, and cannot tell one person from another.
眼色, yǎn sè, an eye for judging people; 認不得, rèn bu de, unable to tell one from another. He is not offended, he is correcting her.
As the common saying goes: 'A piss-bladder, though large, has no weight; a steelyard-weight, though small, presses a thousand catties.'
The comeback is two market-stall proverbs, and they are still funny. 尿泡, suī pāo, a bladder — big and empty. Both characters take their coarse Ming vernacular readings here, suī and pāo, where standard modern Chinese would say niào pào. Against it the 秤鉈, chèng tuó, the little sliding weight on a steelyard balance, which will hold down a thousand catties. 斤兩, jīn liǎng, "catties and taels," is simply the word for weight.
Those two are all show and no use: they breast the wind as they walk, they waste cloth on their clothes, they are hollow-hearted as a banked fire, soft at the crown and weak in the loins, and eat their food to no purpose.
Then he itemises his brothers, and the list is merciless. 相貌, xiàng mào, outward appearance; 抗風, kàng fēng, to breast the wind — they are so broad they have to push through it. 種火心空, zhǒng huǒ xīn kōng, hollow inside like a banked ember; 吃食無功, chī shí wú gōng, they eat to no good end. Wasting cloth on their clothes is the sort of complaint a household accountant would make.
But old Sun of ours, small though he is, is small and solid."
咱, zán, "us" — used familiarly of himself, the way a man says "yours truly." 斤節, jīn jié, compact and substantial, all in one solid piece. The whole argument in four characters: small, yes; slight, no.
Wukong's rejoinder turns the mockery over with a pair of marketplace proverbs — the empty bladder set against the little steelyard weight — answering scorn with wit in the plain coin of folk wisdom. He then repays her in kind, cataloguing his brothers' bulk as sheer useless show, breasting the wind, wasting cloth, hollow within, before claiming for himself the compact solidity the proverbs celebrate.
The passage is a fine instance of the novel's delight in common speech: its greatest hero argues in the idiom of pedlars and market stalls, and vindicates himself not by any display of power but by a homely truth about weight and worth. The comedy of scale — the small thing containing the mighty one — is the governing principle of Wukong's character, and here he states it himself, in the language of the street.