In these examples, top and bottom padding are set to 0, so the block height = line height.

The inline text (set to a larger font-size) inherits the line-height as a factor of the largest font-size i.e. the line height will expand to reflect the largest font on the line.
Line-height: "normal" (set in mPDF by default as 1.33).

Line-height: 2.0 When using relative line-heights, the text is aligned vertically so that the centre-line of the line goes throught the middle of the largest font.

Line-heights set as a percentages are computed on the base font-size, and are then inherited and treated the same as absolute lengths. This is also true for "em" values. The line-height of this line is set as 200% of the paragraph font-size (10pt).
When using absolute line-heights, the text is aligned vertically so that the centre-line of the line goes throught the middle of the base font.
This means that as far as possible, multiple lines will remain equally spaced
Line-height: 200%

If the line includes a font-size greater than 1.6 times the computed line-height, then the text baseline is dropped so that the text will approximately fit within the line-height.
Line-height: 2em

If the line includes a font-size greater than 2 times the computed line-height, then the line-height is increased to accommodate the larger fontsize.
Line-height: 2em