Let's leave aside the Googles of this world, and focus on mid and small companies (as you asked.) I think one of the cores to answering this is funding- as in

Let's leave aside the Googles of this world, and focus on mid and small companies (as you asked.) I think one of the cores to answering this is funding- as in "where do the funds for your job come from?" So a VC funded startup has a model with a very small number of customers (ie VCs). Failure to make a sale (round of investment) means layoffs. Businesses which have a small number of valuable customers are in the same boat. If Kodak is your big (only) customer and Kodak goes under... By contrast, say, a healthy business with positive cash flow takes some of that cash to fund a new Sales position. (Assuming competence as you asked) that job is pretty safe. Yes markets fluctuate, yes external events can happen (like Covid or Tarrifs) but hopefully you don't succumb to that. Or take a small company, excess cash flow, decides to branch into a new space. Jobs in that new space are vulnerable until returns start flowing. Once cash flow increases to cover costs the jobs there become a lot more secure. Lastly I'd say that speed of growth is directly correlated to speed of shrinking. Businesses that grow hundreds of jobs a week (in the good times) are more likely to shed hundreds of jobs a week in the bad times.