This is correct. Especially with cold extraction. You need more tea.
Heat extraction will produce faster results, and you can use less tea. However some people feel this produces a bitter result. Yet the key is to know when to pull the tea leaves out. Just as making a cup of tea results in bitter tea if you leave the leaves sit in hot water too long... the same will happen with either hot or cold extraction in pg or vg.
I use heat extraction and about one and a half tablespoons of loose tea per 30 ml of pg. This will yield about 12-15 ml of extract. I use my crock pot and a beaker in a hot water bath at 125° for about 45 mins. Then triple strain
thru coffee filters.
Ive yet to make a bitter batch but where I may not taste bitter, you may so adjust accordingly.