\par\bigskip and the {parskip} package

\par\bigskip and the {parskip} package

I read that if one uses the parskip package then there is no need to add \par\bigskip to separate paragraphs; it is enough to move the new sentence to new line with Enter. That's true but at some locations I'm getting a very big skip although I just made a skip by moving the paragraph to a new line with the Enter button. And if I for example change the linespread from 1.8 to 1.6 I'll get just at some locations bigger skips. So how to use the package correctly?
\documentclass[12pt]{book}
\usepackage[german]{babel}
\usepackage[utf8]{inputenc}
\usepackage{parskip}
\usepackage{amsmath}
\usepackage{graphicx}
\usepackage{caption}
\usepackage{subcaption}
\usepackage[top=3cm,bottom=1.5cm,left=3cm,right=2cm,asymmetric]{geometry}
\usepackage{booktabs,dcolumn}
\usepackage[figureposition=bottom]{caption}
\usepackage{fancyhdr}
\numberwithin{equation}{chapter}
\fancyhf{}
\fancyhead[C]{\leftmark}
\fancyhead[LE,RO]{\thepage}
\pagestyle{fancy}
\renewcommand{\headrulewidth}{0.4pt}
\renewcommand{\chaptermark}[1]{
 \markboth{#1}{}}
\newcolumntype{d}[1]{D{.}{.}{#1}}
\newcommand*{\mc}[1]{\multicolumn{1}{c}{#1}}
\newcommand*{\mcTwo}[1]{\multicolumn{2}{c}{#1}}
\linespread{1.8}
\setlength\parindent{0pt}
\begin{document}