The SharePoint Online native layout use the HTML5 and CSS3 best pratices to provide a responsive layout, this result that you see is associated the SharePoint List Column grid from the SharePoint native branding layout, and during importing, the selected column type used in your import is a single-line-text (that support with limit of 255 characters).
You can try any solutions not supported by Microsoft, like include a CSS rule to increase the width of columns, or change XSLT schema to forcing the changes, but prevent the negative impacts on all other Lists that consumes AllItems.aspx (based on your example) or other template files that you pretend modifie to satisfy your requirements.
But you may try to access SharePoint List settings and change column type that have long text to a multiline-text, with this change, the AllItems.aspx Grid will apply break-line at column type multiline-text.