You are allocating quite a lot more than 25 centimeters: no reasonable paper format allows it.
I'd avoid vertical rules and double rules: they add nothing and make for hard reading.
Using siunitx
we ensure correct output of numbers (the comma should not have a space behind it).
The very long headers are conveniently split into two lines.
I used \addlinespace
because of the fractions.
\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[dutch]{babel}
\usepackage{booktabs}
\usepackage{siunitx}
\usepackage{lipsum} % for mock content
\sisetup{
output-decimal-marker={,}
}
\begin{document}
\lipsum[1][1-5]
\begin{table}[htp!]
\centering
\begin{tabular}{
@{}
l
S[table-format=4.0]
c
S[table-format=3.2]
S[table-format=3.1]
S[table-format=1.3]
@{}
}
\toprule
Buis & {Lengte} & Debiet & {Diameter} & {Handelsmaat} & {Drukverval} \\
& {(\unit{m})} & (\unit{m^3/h}) & {(\unit{mm})} & {diameter (\unit{mm})} & {(\unit{m})} \\
\midrule
A-R & 2360 & \num{1242,50} & 548,81 & 570 & 0,063 \\ \addlinespace
A-B & 50 & $\frac{3479}{6}$ & 374,91 & 407 & 0,002 \\ \addlinespace
A-M & 200 & \num{124,25} & 173,55 & 180,8 & 0,017 \\ \addlinespace
A-I & 750 & $\frac{5467}{12}$ & 332,32 & 361,8 & 0,032 \\ \addlinespace
B-C & 150 & $\frac{994}{3}$ & 283,40 & 285 & 0,008 \\ \addlinespace
B-H & 425 & $\frac{497}{3}$ & 200,40 & 203,4 & 0,032 \\ \addlinespace
C-D & 250 & $\frac{2485}{12}$ & 224,05 & 226,2 & 0,017 \\ \addlinespace
C-G & 425 & $\frac{497}{12}$ & 100,20 & 113 & 0,058 \\ \addlinespace
D-E & 700 & $\frac{497}{6}$ & 141,70 & 144,6 & 0,034 \\ \addlinespace
D-F & 500 & $\frac{497}{12}$ & 100,20 & 113 & 0,068 \\ \addlinespace
F-G & 275 & $\frac{497}{12}$ & 100,20 & 113 & 0,037 \\ \addlinespace
G-H & 325 & $\frac{497}{6}$ & 141,70 & 144,6 & 0,034 \\ \addlinespace
I-J & 325 & $\frac{994}{3}$ & 283,40 & 285 & 0,017 \\ \addlinespace
I-L & 150 & $\frac{497}{12}$ & 100,20 & 113 & 0,020 \\ \addlinespace
J-K & 300 & $\frac{497}{3}$ & 200,40 & 203,4 & 0,023 \\ \addlinespace
J-O & 200 & $\frac{497}{6}$ & 141,70 & 144,6 & 0,021 \\ \addlinespace
K-N & 200 & $\frac{497}{6}$ & 141,70 & 144,6 & 0,021 \\ \addlinespace
L-M & 750 & $\frac{497}{12}$ & 100,20 & 113 & 0,101 \\
\bottomrule
\end{tabular}
\caption{De maten van de buizen in het distributiesysteem}
\label{table:distributiesysteem}
\end{table}
\lipsum[2]
\end{document}

Switching to the slashed form allows for a more compact (and more readable) table.
\begin{tabular}{
@{}
l
S[table-format=4.0]
c
S[table-format=3.2]
S[table-format=3.1]
S[table-format=1.3]
@{}
}
\toprule
Buis & {Lengte} & Debiet & {Diameter} & {Handelsmaat} & {Drukverval} \\
& {(\unit{m})} & (\unit{m^3/h}) & {(\unit{mm})} & {diameter (\unit{mm})} & {(\unit{m})} \\
\midrule
A-R & 2360 & \num{1242,50} & 548,81 & 570 & 0,063 \\
A-B & 50 & $3479/6$ & 374,91 & 407 & 0,002 \\
A-M & 200 & \num{124,25} & 173,55 & 180,8 & 0,017 \\
A-I & 750 & $5467/12$ & 332,32 & 361,8 & 0,032 \\
B-C & 150 & $994/3$ & 283,40 & 285 & 0,008 \\
B-H & 425 & $497/3$ & 200,40 & 203,4 & 0,032 \\
C-D & 250 & $2485/12$ & 224,05 & 226,2 & 0,017 \\
C-G & 425 & $497/12$ & 100,20 & 113 & 0,058 \\
D-E & 700 & $497/6$ & 141,70 & 144,6 & 0,034 \\
D-F & 500 & $497/12$ & 100,20 & 113 & 0,068 \\
F-G & 275 & $497/12$ & 100,20 & 113 & 0,037 \\
G-H & 325 & $497/6$ & 141,70 & 144,6 & 0,034 \\
I-J & 325 & $994/3$ & 283,40 & 285 & 0,017 \\
I-L & 150 & $497/12$ & 100,20 & 113 & 0,020 \\
J-K & 300 & $497/3$ & 200,40 & 203,4 & 0,023 \\
J-O & 200 & $497/6$ & 141,70 & 144,6 & 0,021 \\
K-N & 200 & $497/6$ & 141,70 & 144,6 & 0,021 \\
L-M & 750 & $497/12$ & 100,20 & 113 & 0,101 \\
\bottomrule
\end{tabular}

array
package andwc{5cm}
notc{5cm}