To reduce the horizontal space between fields in a flex layout using Angular Material, you can adjust the spacing using the mat-form-field class and the mat-grid-list component.

Here are the steps you can follow:

Set the mat-grid-list component to use a fixed number of columns, like this:

<mat-grid-list cols="2" rowHeight="2:1">

Use the mat-form-field class to style your input fields, like this:

<mat-form-field class="example-form-field">
  <input matInput placeholder="First Name">
</mat-form-field>

Define a custom style for the mat-form-field class, like this:

.example-form-field {
  margin-right: 8px;
}

This will set a margin of 8 pixels between each input field.