It is helpful to understand that a lot of language contained in statutes do not create private law obligations (i.e. things that you could sue another private individual over) or generally applicable public law obligations (e.g. crimes and tax laws applicable to ordinary individuals).
I have a set of state statutes in my office that can no longer fit on a single shelf of my book case, and my unannotated version of the tax code runs two thick volumes, plus six volumes of regulations.
But, 95%+ of the lawyers in the state use less than 10% of the statutes by page length, on any regular basis, and I probably use only 10% of the tax code and 5% of the tax regulations despite having an extremely broad tax practice in terms of the variety of subjects I deal with compared to the average lawyer who does tax work.
A very large share of statutory language is a blueprint setting forth the structure and organizational (constitution-like) operating rules of a vast governmental bureaucracy, and don't impose either private law or public law obligations on any private individuals.
Another very large share of statutory language creates legal obligations related to the regulation of very complicated activities that have application to a very small subset of people.
For example, there are lengthy, detailed and complicated laws that explain what public utilities that operate power plants must do in order to protect the environment from pollution.
Similarly, the tax code contains lengthy and difficult long sections pertaining to the tax treatment of mergers and breakups into multiple companies of publicly held companies, and the tax treatment of related companies in multinational groups of affiliated companies, that have to be very detailed in an effort to prevent taxes from being avoided by restructuring a transaction in a way that wouldn't otherwise be considered or make economic sense. These provisions are added to piecemeal every time some new loophole is discovered.
The tax code also has sections almost as long as the entire tax code applicable to ordinary individuals who earn wages and salaries to cover specialty topics like how to determine the taxable income of an insurance company.
Almost every specialized and technical industry has its own lengthy and detailed set of state and federal laws regulating them in multiple respects. There are laws regulating credit reporting agencies, laws regulating bond rating agencies, laws regulating banks, regulating savings and loans, regulating credit unions, regulating financial institutions that lend to credit unions, establishing and regulating the federal reserve, regulating mortgage financing, regulating automobile financing, regulating export-import financing, and so on and so on.
Probably half of the legislative output in any given year establishes that year's governmental budget and appropriations for the current fiscal year, agency by subagency by program by earmark by footnote.
Most long laws, like the ACA or Tax Code, are really just a collection of a lot of little laws.
The ACA has a law establishing a federal health care exchange, a law establishing state health care exchanges, a law creating new departments and sub-departments within the department of health and human services with all of the details about civil service rules, procurement and establishing regulations that that entails, a law governing how insurance companies can set rates and decide to allow customers to buy insurance, a law governing how much money insurance companies have to spend on patient care, a law on what insurance policies have to cover, a law on Medicaid expansion, a law on controlling government program spending on health care, a law on gathering information about health care, a law creating a new payroll tax, a tax law governing credits and penalties for people who do and do not pay health insurance, a law on medical privacy, a law on professional liability, a law on medical education standards, a law on funding for medical education, etc.
Many of the individual sub-laws in the ACA are quite manageable in length, put if you create 30-40 different sub-laws with an average of 25-30 pages each, before you know it, you've got a 900 page long omnibus bill.
The tax code covers not just income taxes, but gift and estate taxes, payroll taxes, gas taxes, bow and arrow taxes, firearm taxes, corporate taxes, partnership taxes, cooperative taxes, non-profit sector regulation, pension taxation, expatriation taxes, tax penalties, and the organization of the Internal Revenue Service. Again, there are lots of little sub-laws in there, many of which have little relevant to the vast majority of people, that cover detailed situations and the administration of other parts of the law.