TCPDF assigned default-values to the cell-paddings, which can be overwritten:
$pdf->setCellPaddings( $left = '', $top = '', $right = '', $bottom = '');
Or, with using the same padding for all sides:
$pdf->setCellPadding( $padding );
See the documentation here: http://www.tcpdf.org/doc/code/classTCPDF.html#a374b24751bf76e4ca5ba7694f87ec2c1
Update 22.04.2020: https://tcpdf.org/docs/srcdoc/TCPDF/class-TCPDF/#_SetCellPadding
Update 20.08.2020: https://tcpdf.org/docs/srcdoc-copy/TCPDF/class-TCPDF/#_SetCellPadding