You can try to put table width=100% tags around the content of those problematic pages. E.g. in pop_profile.asp,
<!--#INCLUDE FILE="config.asp" -->
<!--#INCLUDE FILE="inc_functions.asp" -->
<% if not(strUseExtendedProfile) then %>
<!--#INCLUDE FILE="inc_top_short.asp" -->
<% else %>
<!--#INCLUDE FILE="inc_top.asp" -->
<table width=100%><tr><td>
<%
end if
'....
... and (at the end)
if not(strUseExtendedProfile) then
%>
<!--#INCLUDE FILE="inc_footer_short.asp" -->
<% else
%>
</td></tr></table>
<%
WriteFooter
end if %>
Hope that helps. Looking at search.asp can also give you some hints how to deal with small page content as this one is displayed correctly in your forum (it uses a table with 100% width too).
I like much your forum design by the way.
Barbara