Hi Bob -
That is the size of the viewport, not of the geometry that is seen through the viewport. The viewBox information that is stated before that size tells a browser which part and at which scale the geometry will be rendered at.
When you open that file in Inkscape and check the document properties, you will see this:
Inkscape is scaling the geometry by 0.132292.
If you look at two of the longest curves in the SVG code and check their coordinates, you will see that these match with what Rhino is drawing.
<?xml version="1.0"?>
<svg xmlns="http://www.w3.org/2000/svg" version="1.0" viewBox="275.5789489746094 598.451416015625 765.0440673828125 761.4630126953125" width="101.20895474751791mm" height="100.73521105448405mm">
<style id="svg-style">.centerline { stroke: green; fill: none; stroke-width: 3px; }
</style>
<path id="path-145" class="centerline" d="M894 1082 C918 1082 942 1082 966 1081 C967 1081 969 1080 971 1080 C977 1080 984 1080 991 1080 C994 1080 998 1080 1001 1081 C1001 1081 1002 1082 1002 1082 C1003 1083 1004 1083 1006 1083 C1006 1083 1007 1082 1008 1083 C1009 1083 1009 1084 1010 1085 C1013 1086 1015 1086 1018 1087 C1020 1088 1022 1090 1023 1091 C1026 1094 1029 1096 1030 1100 C1033 1106 1035 1112 1036 1119 C1037 1123 1037 1128 1037 1132 C1037 1158 1036 1184 1036 1210 C1036 1241 1037 1272 1037 1303 C1037 1307 1037 1312 1036 1316 C1036 1320 1035 1324 1033 1328 C1030 1333 1026 1338 1022 1342 C1020 1344 1019 1346 1017 1347 C1010 1349 1003 1351 997 1353 C994 1353 992 1353 989 1353 C974 1353 959 1353 944 1353 C928 1353 912 1352 897 1352 C858 1352 819 1352 781 1352 C764 1352 747 1350 731 1350 C718 1350 704 1351 691 1351 C684 1351 676 1350 668 1350 C663 1350 658 1350 653 1350 C651 1350 649 1349 647 1349 C640 1349 633 1349 627 1349 C625 1349 624 1350 622 1351"/>
<path id="path-152" class="centerline" d="M297 608 C300 607 303 605 306 604 C309 603 313 601 317 601 C325 600 332 599 340 599 C373 599 407 600 440 599 C457 599 473 598 490 598 C496 598 502 598 508 599 C511 599 513 599 516 600 C519 601 522 602 526 604 C532 607 537 611 543 615 C544 616 544 617 545 618 C547 622 549 626 549 630 C551 637 551 644 551 651 C552 782 552 913 553 1045 C553 1048 553 1051 553 1053 C554 1056 554 1058 555 1059 C557 1065 565 1070 563 1076 C561 1080 555 1071 552 1068 C550 1065 550 1062 549 1059 C548 1055 548 1051 548 1047 C547 914 545 781 546 649 C546 645 549 634 551 638 C553 643 550 649 549 655"/>
</svg>
-wim