A good question, which set me off to find some answers.
Some snippets from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5441126/
The first states that TG-rich lipoproteins are converted into sdLDL…
When plasma TG levels were normal, the liver secreted primarily apoE-containing TG-rich VLDL that was rapidly removed from the circulation. In hypertriglyceridemia, however, the balance was shifted towards apoC-III-containing TG-rich lipoproteins that had longer circulation times and were converted into sdLDL…
Here’s a passage that speaks to “association” between sdLDL and trigs - which we agree is often a toothless claim…
As the accumulating evidence points to the important role of sdLDL in the development of atherosclerosis and CVD, many studies focus on improving the lipid profile. The predominance of sdLDL is associated with the elevated TG and decreased HDL levels
This next paper also contains passages that are more on point:
It’s a dense read and acknowledges that the origin of sdLDL “remains to be elucidated” - but here’s a detailed passage that might be of some interest:
Cholesteryl ester transfer protein (CETP) can transfer TG to sdLDL particles that will be further delipidated by HL, resulting in the generation of smaller particles. This theory advocates the distinct metabolic pathway for sdLDL from liver-secreted precursors and is supported by the results of an interventional human study that demonstrated an inverse correlation between LDL I and LDL III and between LDL II and LDL IV (note: LDL-IV in this paper refers to sdLDL). As a consequence of step-wise modification, sdLDL particles have altered chemical contents, containing decreased amounts of phospholipids (as measured based on apolipoprotein B content), as well as free cholesterol and cholesterol ester, while TG contents remain unaltered.
This last sentence suggests that it’s the loss of other compounds that leave the sdLDL smaller and denser, even while shuttling around triglycerides.
That may address your question about how they get smaller and denser?