The Table style is defined in the stylesheet named PublicationDefault.nb, which is referenced by the various article and book stylesheets. The key option in effect here is this setting for column item size:
GridBoxItemSize->{"Columns" -> {{
Scaled[0.503]}}, "ColumnsIndexed" -> {}, "Rows" -> {{1.}}, "RowsIndexed" -> {}}
making each column 50% of the window width. This is really only appropriate for 2 column tables, and should probably be reconfigured to something a little more practical. To override the setting, you can apply the following at the cell level:
GridBoxOptions->{GridBoxItemSize->{"Columns" -> {}}}