Why does font weight affect font width?

[CodingOctocat]

I attempted to highlight and bold specific content blocks, but I found that FontWeight.Bold causes the font to expand in width. This ultimately leads to a poor user experience: if a sentence occupies exactly one line, bolding it may cause the last word to wrap to the next line.

public class TextMarker : TextSegment
{
    public Brush? BackgroundColor { get; set; }

    public FontWeight? FontWeight { get; set; }
}

public class TextMarkerService : IBackgroundRenderer, IVisualLineTransformer
{
    private readonly TextSegmentCollection<TextMarker> _markers;

    public KnownLayer Layer => KnownLayer.Selection;

    public TextMarkerService(TextDocument document)
    {
        _markers = new TextSegmentCollection<TextMarker>(document);
    }

    public void Clear()
    {
        _markers.Clear();
    }

    public TextMarker Create(int start, int length)
    {
        var marker = new TextMarker {
            StartOffset = start,
            Length = length
        };
        _markers.Add(marker);
        return marker;
    }

    public void Draw(TextView textView, DrawingContext drawingContext)
    {
        if (_markers.Count == 0)
        {
            return;
        }

        textView.EnsureVisualLines();

        foreach (var marker in _markers)
        {
            if (marker.BackgroundColor is null)
            {
                continue;
            }

            foreach (var r in BackgroundGeometryBuilder.GetRectsForSegment(textView, marker))
            {
                drawingContext.DrawRectangle(marker.BackgroundColor, null, new Rect(r.Location, r.Size));
            }
        }
    }

    public void Transform(ITextRunConstructionContext context, System.Collections.Generic.IList<VisualLineElement> elements)
    {
        foreach (var marker in _markers)
        {
            if (marker.FontWeight is null)
            {
                continue;
            }

            int markerStart = marker.StartOffset;
            int markerEnd = marker.EndOffset;

            foreach (var element in elements)
            {
                int elementStart = context.VisualLine.FirstDocumentLine.Offset + element.RelativeTextOffset;
                int elementEnd = elementStart + element.DocumentLength;

                if (elementEnd <= markerStart || elementStart >= markerEnd)
                {
                    continue;
                }

                var props = element.TextRunProperties;
                element.TextRunProperties.SetTypeface(new Typeface(
                    props.Typeface.FontFamily,
                    props.Typeface.Style,
                    marker.FontWeight.Value,
                    props.Typeface.Stretch));
            }
        }
    }
}