I created a simple project with TableView containing 10 000 000 rows and vertical scrollbar.
The issue that the more you scroll it down, the more extra spaces between rows appear.

If you change delegate height to 15 or 31 - extra spaces dissapear and TableView looks as expected until you change rows count to about 17 000 000 or more.

If you try to set rows count to more than 100 000 000, warning appears:
"Model size of 200000000 is bigger than the upper limit 100000000", that implies that TableView is designed to process up to that limit of rows.

Exacly the same happens with ListView.

The issue is reproduced on Windows 7/10 and Ubuntu 16.

import QtQuick 2.12
import QtQuick.Window 2.12
import QtQuick.Controls 2.5

Window {
    visible: true
    width: 640
    height: 480

    TableView {
        anchors.fill: parent
        rowSpacing: 1

        model: 1000000

        delegate: Rectangle {
            color: "green"
            implicitWidth: 100
            implicitHeight: 20

            Text {
                text: index
            }
        }

        ScrollBar.vertical: ScrollBar {
            minimumSize: 0.1;
        }
    }
}