As you say, the first thing any reader has to reckon with in reading Master of the Senate is its size. It's not just the sheer length of 1,167 pages, nor simply that those 1,167 pages cover only Lyndon Johnson's Senate career. (How will Caro ever squeeze the 1960 presidential race, Kennedy's presidency and LBJ's own presidency into just one volume? How many subdivisions do you predict? Let's take bets.)
No, more than the book's actual length, what struck me about this mammoth work is that Caro seems determined to give us a biography commensurate with what he considers Johnson's own outsized personality and immense historical significance. The monumental size of this trilogy-plus, in other words, is intrinsic to its portrait of Johnson—indeed to its argument about Johnson's importance in American history. More on which below.
Many of this book's virtues are obvious and will be widely praised: the research; the prose (which, despite lapsing into cliché, is redeemed because the style is so palpably Caro's own); the fine feel for the details of Senate politics. But there are other virtues, too, that those who dismiss the book as mere "story-telling" might overlook.
For example, Caro's 100 pages recounting the history of the Senate is surely excessive. But it serves an important purpose. Caro appreciates the role that history itself plays in the Senate, the way that the body's traditions (and, compared to the House, its undemocratic nature) served the segregationist South even after that region's racial attitudes had become anathema to most Americans. This focus on the Senate as a body—a focus that shapes the book—is a shrewd interpretive move. It goes a long way toward explaining why the civil rights cause languished even after Harry Truman's presidency and Brown v. Board of Education. Simply put, the Senate's rules and traditions allowed it to be controlled by a regional minority.
There are other merits, too, likely to be unsung: Caro's grasp of the rich ideological variation that used to exist within both parties; his understanding of the array of weapons in Johnson's arsenal (flattery, intimidation, self-pity, fake high-mindedness, etc.); his ability to keep you following along as he descends into legislative minutiae. If we don't discuss these qualities later, I want to credit them now.
But to address some of your questions: I do think Caro brilliantly captures LBJ's gargantuan appetites, and I do think he's more than a physically large man who liked to drink and have sex. The oft-used "Rabelasian" label applies. There's a wonderful passage—really, the entirety of Chapter 27—of Johnson's eating and drinking habits circa 1955, just before his heart attack, in which Caro vividly shows us Johnson shoveling down platters of black-eyed peas and tapioca pudding, gulping tumblers of Scotch in quick succession, chain-smoking "like a man who had a date with a firing squad" (Russell Baker's description). This behavior, Caro convincingly shows, was bound up with LBJ's bottomless ambition, his drive to push harder all the time. Caro's disquisitions on Johnson's crudeness also, I think, show the connections among size, appetite, and ambition. LBJ used to refer to his sex organ as "Jumbo," and sometimes, after urinating, would start talking to a colleague without zipping up, instead simply beginning the conversation: "Have you ever seen anything as big as this?" I'll let the psychoanalysts take it from there. (By the way, I had trouble finding this last passage but then discovered in the index under "Johnson, Lyndon B., sexual exhibitionism," a few lines below "Johnson, Lyndon B., scratching habit.")
Now you might ask: Does a physically big Johnson make for a historically big Johnson? Or as you wondered, does LBJ deserve this jumbo biography? I tend to think not. After all, Robert Dallek's excellent two-volume work (total pages: 1,500) certainly gives Johnson his due; and when I did a little cross-checking, I found that some of the "revelations" that reviewers have hailed as "new" in Caro's work—LBJ's secret involvement with the Southern Caucus (the unified bloc of Southerners who plotted anti-civil rights strategy); the McCarthy-like attack on Leland Olds that you mentioned—were also in Dallek's book, albeit in condensed form. Don't you agree that 1,167 pages on the Senate years, as enjoyable to read as they are, amount to overkill? Does it perhaps say more about Caro's grandiose ambitions, his Ahab-like quest to master the master of the Senate, than about Johnson himself?
I'll leave it there for today. I'm curious to know your thought about Caro and his outsized ambitions, his Johnson-like fascination with power (is there a hunger for power in his attempt to set forth a definitive life of someone whom I think he does hate, but in a love-hate kind of way?). Maybe we can also discuss of the civil rights bill that dominates the thrilling climax of this book—which is also where I think Caro's claims are historically the most problematic.
Over to you,