You can scale lineweights proportionately in a layout with the plot scale.
Typically, lineweights specify the line width of plotted objects and are plotted with the line width size regardless of the plot scale. Most often, you use the default plot scale of 1:1 when plotting a layout. However, if you want to plot an E-size layout that is scaled to fit on an A-size sheet of paper, for example, you can specify lineweights to be scaled in proportion to the new plot scale.
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The lineweights in the current layout are scaled in proportion to the designated plot scale. When you are working in the Model tab, this option is not available.